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Table of Contents
MySQL supports a number of data types in several categories: numeric types, date and time types, and string (character) types. This chapter first gives an overview of these data types, and then provides a more detailed description of the properties of the types in each category, and a summary of the data type storage requirements. The initial overview is intentionally brief. The more detailed descriptions later in the chapter should be consulted for additional information about particular data types, such as the allowable formats in which you can specify values.
MySQL also supports extensions for handing spatial data. Chapter 16, Spatial Extensions, provides information about these data types.
Several of the data type descriptions use these conventions:
MM is the total
number of digits. For string types, M
is the maximum length. The maximum allowable value of
M depends on the data type.
DM–2.
Square brackets (‘[’ and
‘]’) indicate optional parts of
type definitions.
A summary of the numeric data types follows. For additional information, see Section 11.2, “Numeric Types”. Storage requirements are given in Section 11.5, “Data Type Storage Requirements”.
M indicates the maximum display
width. The maximum legal display width is 255. Display width is
unrelated to the storage size or range of values a type can
contain, as described in Section 11.2, “Numeric Types”.
If you specify ZEROFILL for a numeric column,
MySQL automatically adds the UNSIGNED
attribute to the column.
SERIAL is an alias for BIGINT
UNSIGNED NOT NULL AUTO_INCREMENT UNIQUE.
SERIAL DEFAULT VALUE in the definition of an
integer column is an alias for NOT NULL AUTO_INCREMENT
UNIQUE.
Warning: When you use
subtraction between integer values where one is of type
UNSIGNED, the result is unsigned. See
Section 12.8, “Cast Functions and Operators”.
A bit-field type. M indicates the
number of bits per value, from 1 to 64. The default is 1 if
M is omitted.
This data type was added in MySQL 5.0.3 for
MyISAM, and extended in 5.0.5 to
MEMORY, InnoDB, and
BDB. Before 5.0.3, BIT
is a synonym for TINYINT(1).
TINYINT[(
M)] [UNSIGNED]
[ZEROFILL]
A very small integer. The signed range is
-128 to 127. The
unsigned range is 0 to
255.
These types are synonyms for TINYINT(1).
A value of zero is considered false. Non-zero values are
considered true.
In the future, full boolean type handling will be introduced in accordance with standard SQL.
SMALLINT[(
M)] [UNSIGNED]
[ZEROFILL]
A small integer. The signed range is
-32768 to 32767. The
unsigned range is 0 to
65535.
MEDIUMINT[(
M)]
[UNSIGNED] [ZEROFILL]
A medium-sized integer. The signed range is
-8388608 to 8388607.
The unsigned range is 0 to
16777215.
INT[(
M)] [UNSIGNED]
[ZEROFILL]
A normal-size integer. The signed range is
-2147483648 to
2147483647. The unsigned range is
0 to 4294967295.
INTEGER[(
M)] [UNSIGNED]
[ZEROFILL]
This type is a synonym for INT.
BIGINT[(
M)] [UNSIGNED]
[ZEROFILL]
A large integer. The signed range is
-9223372036854775808 to
9223372036854775807. The unsigned range
is 0 to
18446744073709551615.
Some things you should be aware of with respect to
BIGINT columns:
All arithmetic is done using signed
BIGINT or DOUBLE
values, so you should not use unsigned big integers
larger than 9223372036854775807 (63
bits) except with bit functions! If you do that, some of
the last digits in the result may be wrong because of
rounding errors when converting a
BIGINT value to a
DOUBLE.
MySQL can handle BIGINT in the
following cases:
When using integers to store large unsigned values
in a BIGINT column.
In
MIN(
or
col_name)MAX(,
where col_name)col_name refers to
a BIGINT column.
When using operators (+,
-, *, and so
on) where both operands are integers.
You can always store an exact integer value in a
BIGINT column by storing it using a
string. In this case, MySQL performs a string-to-number
conversion that involves no intermediate
double-precision representation.
The -, +, and
* operators use
BIGINT arithmetic when both operands
are integer values. This means that if you multiply two
big integers (or results from functions that return
integers), you may get unexpected results when the
result is larger than
9223372036854775807.
FLOAT[(
M,D)]
[UNSIGNED] [ZEROFILL]
A small (single-precision) floating-point number. Allowable
values are -3.402823466E+38 to
-1.175494351E-38, 0,
and 1.175494351E-38 to
3.402823466E+38. These are the
theoretical limits, based on the IEEE standard. The actual
range might be slightly smaller depending on your hardware
or operating system.
M is the total number of decimal
digits and D is the number of
digits following the decimal point. If
M and
D are omitted, values are stored
to the limits allowed by the hardware. A single-precision
floating-point number is accurate to approximately 7 decimal
places.
UNSIGNED, if specified, disallows
negative values.
Using FLOAT might give you some
unexpected problems because all calculations in MySQL are
done with double precision. See
Section A.5.7, “Solving Problems with No Matching Rows”.
DOUBLE[(
M,D)]
[UNSIGNED] [ZEROFILL]
A normal-size (double-precision) floating-point number.
Allowable values are
-1.7976931348623157E+308 to
-2.2250738585072014E-308,
0, and
2.2250738585072014E-308 to
1.7976931348623157E+308. These are the
theoretical limits, based on the IEEE standard. The actual
range might be slightly smaller depending on your hardware
or operating system.
M is the total number of decimal
digits and D is the number of
digits following the decimal point. If
M and
D are omitted, values are stored
to the limits allowed by the hardware. A double-precision
floating-point number is accurate to approximately 15
decimal places.
UNSIGNED, if specified, disallows
negative values.
DOUBLE
PRECISION[(,
M,D)]
[UNSIGNED] [ZEROFILL]REAL[(
M,D)]
[UNSIGNED] [ZEROFILL]
These types are synonyms for DOUBLE.
Exception: If the REAL_AS_FLOAT SQL mode
is enabled, REAL is a synonym for
FLOAT rather than
DOUBLE.
FLOAT(
p) [UNSIGNED]
[ZEROFILL]
A floating-point number. p
represents the precision in bits, but MySQL uses this value
only to determine whether to use FLOAT or
DOUBLE for the resulting data type. If
p is from 0 to 24, the data type
becomes FLOAT with no
M or D
values. If p is from 25 to 53,
the data type becomes DOUBLE with no
M or D
values. The range of the resulting column is the same as for
the single-precision FLOAT or
double-precision DOUBLE data types
described earlier in this section.
DECIMAL[(
M[,D])]
[UNSIGNED] [ZEROFILL]
For MySQL 5.0.3 and above:
A packed “exact” fixed-point number.
M is the total number of decimal
digits (the precision) and D is
the number of digits after the decimal point (the scale).
The decimal point and (for negative numbers) the
‘-’ sign are not counted in
M. If
D is 0, values have no decimal
point or fractional part. The maximum number of digits
(M) for
DECIMAL is 65 (64 from 5.0.3 to 5.0.5).
The maximum number of supported decimals
(D) is 30. If
D is omitted, the default is 0.
If M is omitted, the default is
10.
UNSIGNED, if specified, disallows
negative values.
All basic calculations (+, -, *, /) with
DECIMAL columns are done with a precision
of 65 digits.
Before MySQL 5.0.3:
An unpacked fixed-point number. Behaves like a
CHAR column; “unpacked”
means the number is stored as a string, using one character
for each digit of the value. M is
the total number of digits and D
is the number of digits after the decimal point. The decimal
point and (for negative numbers) the
‘-’ sign are not counted in
M, although space for them is
reserved. If D is 0, values have
no decimal point or fractional part. The maximum range of
DECIMAL values is the same as for
DOUBLE, but the actual range for a given
DECIMAL column may be constrained by the
choice of M and
D. If
D is omitted, the default is 0.
If M is omitted, the default is
10.
UNSIGNED, if specified, disallows
negative values.
DEC[(,
M[,D])]
[UNSIGNED] [ZEROFILL]NUMERIC[(,
M[,D])]
[UNSIGNED] [ZEROFILL]FIXED[(
M[,D])]
[UNSIGNED] [ZEROFILL]
These types are synonyms for DECIMAL. The
FIXED synonym is available for
compatibility with other database systems.
A summary of the temporal data types follows. For additional information, see Section 11.3, “Date and Time Types”. Storage requirements are given in Section 11.5, “Data Type Storage Requirements”.
For the DATETIME and DATE
range descriptions, “supported” means that although
earlier values might work, there is no guarantee.
The SUM() and AVG()
aggregate functions do not work with temporal values. (They
convert the values to numbers, which loses the part after the
first non-numeric character.) To work around this problem, you
can convert to numeric units, perform the aggregate operation,
and convert back to a temporal value. Examples:
SELECT SEC_TO_TIME(SUM(TIME_TO_SEC(time_col))) FROMtbl_name; SELECT FROM_DAYS(SUM(TO_DAYS(date_col))) FROMtbl_name;
A date. The supported range is
'1000-01-01' to
'9999-12-31'. MySQL displays
DATE values in
'YYYY-MM-DD' format, but allows you to
assign values to DATE columns using
either strings or numbers.
A date and time combination. The supported range is
'1000-01-01 00:00:00' to
'9999-12-31 23:59:59'. MySQL displays
DATETIME values in 'YYYY-MM-DD
HH:MM:SS' format, but allows you to assign values
to DATETIME columns using either strings
or numbers.
A timestamp. The range is '1970-01-01
00:00:00' to partway through the year
2037.
A TIMESTAMP column is useful for
recording the date and time of an INSERT
or UPDATE operation. By default, the
first TIMESTAMP column in a table is
automatically set to the date and time of the most recent
operation if you do not assign it a value yourself. You can
also set any TIMESTAMP column to the
current date and time by assigning it a
NULL value. Variations on automatic
initialization and update properties are described in
Section 11.3.1.1, “TIMESTAMP Properties as of MySQL 4.1”.
A TIMESTAMP value is returned as a string
in the format 'YYYY-MM-DD HH:MM:SS' with
a display width fixed at 19 characters. To obtain the value
as a number, you should add +0 to the
timestamp column.
Note: The
TIMESTAMP format that was used prior to
MySQL 4.1 is not supported in MySQL 5.0; see
MySQL 3.23, 4.0, 4.1 Reference Manual for information
regarding the old format.
A time. The range is '-838:59:59' to
'838:59:59'. MySQL displays
TIME values in
'HH:MM:SS' format, but allows you to
assign values to TIME columns using
either strings or numbers.
A year in two-digit or four-digit format. The default is
four-digit format. In four-digit format, the allowable
values are 1901 to
2155, and 0000. In
two-digit format, the allowable values are
70 to 69, representing
years from 1970 to 2069. MySQL displays
YEAR values in YYYY
format, but allows you to assign values to
YEAR columns using either strings or
numbers.
A summary of the string data types follows. For additional information, see Section 11.4, “String Types”. Storage requirements are given in Section 11.5, “Data Type Storage Requirements”.
In some cases, MySQL may change a string column to a type
different from that given in a CREATE TABLE
or ALTER TABLE statement. See
Section 13.1.5.1, “Silent Column Specification Changes”.
In MySQL 4.1 and up, string data types include some features that you may not have encountered in working with previous versions of MySQL (prior to 4.1):
Column definitions for many string data types can include a
CHARACTER SET attribute to specify the
character set. (CHARSET is a synonym for
CHARACTER SET.) The
COLLATE attribute specifies a collation
for the the character set. These attributes apply to
CHAR, VARCHAR, the
TEXT types, ENUM, and
SET. For example:
CREATE TABLE t
(
c1 VARCHAR(20) CHARACTER SET utf8,
c2 TEXT CHARACTER SET latin1 COLLATE latin1_general_cs
);
This table definition creates a column named
c1 that has a character set of
utf8 with the default collation for that
character set, and a column named c2 that
has a character set of latin1 and a
case-sensitive collation.
MySQL 5.0 interprets length specifications in character column definitions in character units. (Previously, MySQL interpreted lengths in bytes.)
For CHAR, VARCHAR, and
the TEXT types, the
BINARY attribute causes the column to be
assigned the binary collation of the column character set.
(Previously, BINARY caused a column to
store binary strings.)
Character column sorting and comparison are based on the
character set assigned to the column. (Previously, sorting
and comparison were based on the collation of the server
character set.) For CHAR and
VARCHAR columns, you can declare the
column with a binary collation or the
BINARY attribute to cause sorting and
comparison to use the underlying character code values
rather than a lexical ordering.
Chapter 10, Character Set Support, provides additional information about use of character sets in MySQL.
[NATIONAL] CHAR(
M)
[BINARY | ASCII | UNICODE]
A fixed-length string that is always right-padded with
spaces to the specified length when stored.
M represents the column length.
The range of M is 0 to 255
characters.
Note: Trailing spaces are
removed when CHAR values are retrieved.
Before MySQL 5.0.3, a CHAR column with a
length specification greater than 255 is converted to the
smallest TEXT type that can hold values
of the given length. For example,
CHAR(500) is converted to
TEXT, and CHAR(200000)
is converted to MEDIUMTEXT. This is a
compatibility feature. However, this conversion causes the
column to become a variable-length column, and also affects
trailing-space removal.
In MySQL 5.0.3 and later, if you attempt to set the length
of a CHAR greater than 255, the
CREATE TABLE or ALTER
TABLE statement in which this is done fails with
an error:
mysql>CREATE TABLE c1 (col1 INT, col2 CHAR(500));ERROR 1074 (42000): Column length too big for column 'col' (max = 255); use BLOB or TEXT instead mysql>SHOW CREATE TABLE c1;ERROR 1146 (42S02): Table 'test.c1' doesn't exist
CHAR is shorthand for
CHARACTER. NATIONAL
CHAR (or its equivalent short form,
NCHAR) is the standard SQL way to define
that a CHAR column should use some
predefined character set. MySQL 4.1 and up
utf8 as this predefined character set.
Section 10.3.6, “National Character Set”.
The BINARY attribute is shorthand for
specifying the binary collation of the column character set.
In this case, sorting and comparison are based on numeric
character values.
The ASCII attribute is shorthand for
CHARACTER SET latin1.
The UNICODE attribute is shorthand for
CHARACTER SET ucs2.
The CHAR BYTE data type is an alias for
the BINARY type. This is a compatibility
feature.
MySQL allows you to create a column of type
CHAR(0). This is useful primarily when
you have to be compliant with old applications that depend
on the existence of a column but that do not actually use
its value. CHAR(0) is also quite nice
when you need a column that can take only two values: A
CHAR(0) column that is not defined as
NOT NULL occupies only one bit and can
take only the values NULL and
'' (the empty string).
CHAR
This type is a synonym for CHAR(1).
[NATIONAL] VARCHAR(
M)
[BINARY]
A variable-length string. M
represents the maximum column length. In MySQL
5.0, the range of M
is 0 to 255 before MySQL 5.0.3, and 0 to 65,535 in MySQL
5.0.3 and later. (The actual maximum length of a
VARCHAR in MySQL 5.0 is determined by the
maximum row size and the character set you use. The maximum
effective length starting with MySQL
5.0.3 is 65,532 bytes.)
Note: Before 5.0.3,
trailing spaces were removed when VARCHAR
values were stored, which differs from the standard SQL
specification.
Prior to MySQL 5.0.3, a VARCHAR column
with a length specification greater than 255 was converted
to the smallest TEXT type that could hold
values of the given length. For example,
VARCHAR(500) was converted to
TEXT, and
VARCHAR(200000) was converted to
MEDIUMTEXT. This was a compatibility
feature. However, this conversion affected trailing-space
removal.
VARCHAR is shorthand for
CHARACTER VARYING.
The BINARY attribute is shorthand for
specifying the binary collation of the column character set.
In this case, sorting and comparison are based on numeric
character values.
VARCHAR values are stored using as many
characters as are needed, plus one byte to record the length
(two bytes for columns that are declared with a length
longer than 255).
The BINARY type is similar to the
CHAR type, but stores binary byte strings
rather than non-binary character strings.
The VARBINARY type is similar to the
VARCHAR type, but stores binary byte
strings rather than non-binary character strings.
A BLOB column with a maximum length of
255 (28 – 1) bytes.
A TEXT column with a maximum length of
255 (28 – 1) characters.
A BLOB column with a maximum length of
65,535 (216 – 1) bytes.
An optional length M can be given
for this type. If this is done, MySQL creates the column as
the smallest BLOB type large enough to
hold values M bytes long.
A TEXT column with a maximum length of
65,535 (216 – 1)
characters.
An optional length M can be given
for this type. If this is done, MySQL creates the column as
the smallest TEXT type large enough to
hold values M characters long.
A BLOB column with a maximum length of
16,777,215 (224 – 1) bytes.
A TEXT column with a maximum length of
16,777,215 (224 – 1)
characters.
A BLOB column with a maximum length of
4,294,967,295 or 4GB (232 –
1) bytes. The maximum effective
(permitted) length of LONGBLOB columns
depends on the configured maximum packet size in the
client/server protocol and available memory.
A TEXT column with a maximum length of
4,294,967,295 or 4GB (232 –
1) characters. The maximum effective
(permitted) length of LONGTEXT columns
depends on the configured maximum packet size in the
client/server protocol and available memory.
An enumeration. A string object that can have only one
value, chosen from the list of values
',
value1'',
value2'..., NULL or the
special '' error value. An
ENUM column can have a maximum of 65,535
distinct values. ENUM values are
represented internally as integers.
A set. A string object that can have zero or more values,
each of which must be chosen from the list of values
',
value1'',
value2'... A SET column can
have a maximum of 64 members. SET values
are represented internally as integers.
The DEFAULT
clause in a data type specification indicates a default value
for a column. With one exception, the default value must be a
constant; it cannot be a function or an expression. This means,
for example, that you cannot set the default for a date column
to be the value of a function such as valueNOW()
or CURRENT_DATE. The exception is that you
can specify CURRENT_TIMESTAMP as the default
for a TIMESTAMP column. See
Section 11.3.1.1, “TIMESTAMP Properties as of MySQL 4.1”.
Prior to MySQL 5.0.2, if a column definition includes no
explicit DEFAULT value, MySQL determines the
default value as follows:
If the column can take NULL as a value, the
column is defined with an explicit DEFAULT
NULL clause.
If the column cannot take NULL as the value,
MySQL defines the column with an explicit
DEFAULT clause, using the implicit default
value for the column data type. Implicit defaults are defined as
follows:
For numeric types other than integer types declared with the
AUTO_INCREMENT attribute, the default is
0. For an
AUTO_INCREMENT column, the default value
is the next value in the sequence.
For date and time types other than
TIMESTAMP, the default is the appropriate
“zero” value for the type. For the first
TIMESTAMP column in a table, the default
value is the current date and time. See
Section 11.3, “Date and Time Types”.
For string types other than ENUM, the
default value is the empty string. For
ENUM, the default is the first
enumeration value.
BLOB and TEXT columns
cannot be assigned a default value.
As of MySQL 5.0.2, if a column definition includes no explicit
DEFAULT value, MySQL determines the default
value as follows:
If the column can take NULL as a value, the
column is defined with an explicit DEFAULT
NULL clause. This is the same as before 5.0.2.
If the column cannot take NULL as the value,
MySQL defines the column with no explicit
DEFAULT clause. For data entry, if an
INSERT or REPLACE
statement includes no value for the column, MySQL handles the
column according to the SQL mode in effect at the time:
If strict SQL mode is not enabled, MySQL sets the column to the implicit default value for the column data type.
If strict mode is enabled, an error occurs for transactional tables and the statement is rolled back. For non-transactional tables, an error occurs, but if this happens for the second or subsequent row of a multiple-row statement, the preceding rows will have been inserted.
Suppose that a table t is defined as follows:
CREATE TABLE t (i INT NOT NULL);
In this case, i has no explicit default, so
in strict mode each of the following statements produce an error
and no row is inserted. When not using strict mode, only the
third statement produces an error; the implicit default is
inserted for the first two statements, but the third fails
because DEFAULT(i) cannot produce a value:
INSERT INTO t VALUES(); INSERT INTO t VALUES(DEFAULT); INSERT INTO t VALUES(DEFAULT(i));
See Section 5.2.5, “The Server SQL Mode”.
For a given table, you can use the SHOW CREATE
TABLE statement to see which columns have an explicit
DEFAULT clause.
MySQL supports all of the standard SQL numeric data types. These
types include the exact numeric data types
(INTEGER, SMALLINT,
DECIMAL, and NUMERIC), as
well as the approximate numeric data types
(FLOAT, REAL, and
DOUBLE PRECISION). The keyword
INT is a synonym for
INTEGER, and the keyword DEC
is a synonym for DECIMAL. For numeric type
storage requirements, see Section 11.5, “Data Type Storage Requirements”.
As of MySQL 5.0.3, a BIT data type is available
for storing bit-field values. (Before 5.0.3, MySQL interprets
BIT as TINYINT(1).) In MySQL
5.0.3, BIT is supported only for
MyISAM. MySQL 5.0.5 extends
BIT support to MEMORY,
InnoDB, and BDB.
As an extension to the SQL standard, MySQL also supports the
integer types TINYINT,
MEDIUMINT, and BIGINT. The
following table shows the required storage and range for each of
the integer types.
| Type | Bytes | Minimum Value | Maximum Value |
| (Signed/Unsigned) | (Signed/Unsigned) | ||
TINYINT | 1 | -128 | 127 |
0 | 255 | ||
SMALLINT | 2 | -32768 | 32767 |
0 | 65535 | ||
MEDIUMINT | 3 | -8388608 | 8388607 |
0 | 16777215 | ||
INT | 4 | -2147483648 | 2147483647 |
0 | 4294967295 | ||
BIGINT | 8 | -9223372036854775808 | 9223372036854775807 |
0 | 18446744073709551615 |
Another extension is supported by MySQL for optionally specifying
the display width of an integer value in parentheses following the
base keyword for the type (for example,
INT(4)). This optional display width
specification is used to left-pad the display of values having a
width less than the width specified for the column.
The display width does not constrain the range of values that can be stored in the column, nor the number of digits that are displayed for values having a width exceeding that specified for the column.
When used in conjunction with the optional extension attribute
ZEROFILL, the default padding of spaces is
replaced with zeroes. For example, for a column declared as
INT(5) ZEROFILL, a value of
4 is retrieved as 00004.
Note that if you store larger values than the display width in an
integer column, you may experience problems when MySQL generates
temporary tables for some complicated joins, because in these
cases MySQL assumes that the data fits into the original column
width.
All integer types can have an optional (non-standard) attribute
UNSIGNED. Unsigned values can be used when you
want to allow only non-negative numbers in a column and you need a
larger upper numeric range for the column. For example, if an
INT column is UNSIGNED, the
size of the column's range is the same but its endpoints shift
from -2147483648 and
2147483647 up to 0 and
4294967295.
Floating-point and fixed-point types also can be
UNSIGNED. As with integer types, this attribute
prevents negative values from being stored in the column. However,
unlike the integer types, the upper range of column values remains
the same.
If you specify ZEROFILL for a numeric column,
MySQL automatically adds the UNSIGNED attribute
to the column.
For floating-point data types, MySQL uses four bytes for single-precision values and eight bytes for double-precision values.
The FLOAT and DOUBLE data
types are used to represent approximate numeric data values. For
FLOAT the SQL standard allows an optional
specification of the precision (but not the range of the exponent)
in bits following the keyword FLOAT in
parentheses. MySQL also supports this optional precision
specification, but the precision value is used only to determine
storage size. A precision from 0 to 23 results in a four-byte
single-precision FLOAT column. A precision from
24 to 53 results in an eight-byte double-precision
DOUBLE column.
MySQL allows a non-standard syntax:
FLOAT(
or
M,D)REAL(
or M,D)DOUBLE
PRECISION(.
Here,
“M,D)(”
means than values are displayed with up to
M,D)M digits in total, of which
D digits may be after the decimal
point. For example, a column defined as
FLOAT(7,4) will look like
-999.9999 when displayed. MySQL performs
rounding when storing values, so if you insert
999.00009 into a FLOAT(7,4)
column, the approximate result is 999.0001.
MySQL treats DOUBLE as a synonym for
DOUBLE PRECISION (a non-standard extension).
MySQL also treats REAL as a synonym for
DOUBLE PRECISION (a non-standard variation),
unless the REAL_AS_FLOAT SQL mode is enabled.
For maximum portability, code requiring storage of approximate
numeric data values should use FLOAT or
DOUBLE PRECISION with no specification of
precision or number of digits.
The DECIMAL and NUMERIC data
types are used to store exact numeric data values. In MySQL,
NUMERIC is implemented as
DECIMAL. These types are used to store values
for which it is important to preserve exact precision, for example
with monetary data.
As of MySQL 5.0.3, DECIMAL and
NUMERIC values are stored in binary format.
Previously, they were stored as strings, with one character used
for each digit of the value, the decimal point (if the scale is
greater than 0), and the ‘-’ sign
(for negative numbers). See Chapter 21, Precision Math.
When declaring a DECIMAL or
NUMERIC column, the precision and scale can be
(and usually is) specified; for example:
salary DECIMAL(5,2)
In this example, 5 is the precision and
2 is the scale. The precision represents the
number of significant digits that are stored for values, and the
scale represents the number of digits that can be stored following
the decimal point. If the scale is 0, DECIMAL
and NUMERIC values contain no decimal point or
fractional part.
Standard SQL requires that the salary column be
able to store any value with five digits and two decimals. In this
case, therefore, the range of values that can be stored in the
salary column is from
-999.99 to 999.99. MySQL
enforces this limit as of MySQL 5.0.3. Before 5.0.3, on the
positive end of the range, the column could actually store numbers
up to 9999.99. (For positive numbers, MySQL
5.0.2 and earlier used the byte reserved for the sign to extend
the upper end of the range.)
In standard SQL, the syntax
DECIMAL( is
equivalent to
M)DECIMAL(.
Similarly, the syntax M,0)DECIMAL is equivalent to
DECIMAL(, where
the implementation is allowed to decide the value of
M,0)M. MySQL supports both of these variant
forms of the DECIMAL and
NUMERIC syntax. The default value of
M is 10.
The maximum number of digits for DECIMAL or
NUMERIC is 65 (64 from MySQL 5.0.3 to 5.0.5).
Before MySQL 5.0.3, the maximum range of
DECIMAL and NUMERIC values
is the same as for DOUBLE, but the actual range
for a given DECIMAL or
NUMERIC column can be constrained by the
precision or scale for a given column. When such a column is
assigned a value with more digits following the decimal point than
are allowed by the specified scale, the value is converted to that
scale. (The precise behavior is operating system-specific, but
generally the effect is truncation to the allowable number of
digits.)
As of MySQL 5.0.3, the BIT data type is used to
store bit-field values. A type of
BIT( allows for
storage of M)M-bit values.
M can range from 1 to 64.
To specify bit values,
b' notation
can be used. value'value is a binary value
written using zeroes and ones. For example,
b'111' and b'100000000'
represent 7 and 128, respectively. See
Section 9.1.5, “Bit-Field Values”.
If you assign a value to a
BIT( column that
is less than M)M bits long, the value is
padded on the left with zeroes. For example, assigning a value of
b'101' to a BIT(6) column
is, in effect, the same as assigning b'000101'.
When asked to store a value in a numeric column that is outside
the data type's allowable range, MySQL's behavior depends on the
SQL mode in effect at the time. For example, if no restrictive
modes are enabled, MySQL clips the value to the appropriate
endpoint of the range and stores the resulting value instead.
However, if the mode is set to TRADITIONAL,
MySQL rejects a value that is out of range with an error, and the
insert fails, in accordance with the SQL standard.
In non-strict mode, when an out-of-range value is assigned to an
integer column, MySQL stores the value representing the
corresponding endpoint of the column data type range. If you store
256 into a TINYINT or TINYINT
UNSIGNED column, MySQL stores 127 or 255, respectively.
When a floating-point or fixed-point column is assigned a value
that exceeds the range implied by the specified (or default)
precision and scale, MySQL stores the value representing the
corresponding endpoint of that range.
Conversions that occur due to clipping when MySQL is not operating
in strict mode are reported as warnings for ALTER
TABLE, LOAD DATA INFILE,
UPDATE, and multiple-row
INSERT statements. When MySQL is operating in
strict mode, these statements fail, and some or all of the values
will not be inserted or changed, depending on whether the table is
a transactional table and other factors. For details, see
Section 5.2.5, “The Server SQL Mode”.
The date and time types for representing temporal values are
DATETIME, DATE,
TIMESTAMP, TIME, and
YEAR. Each temporal type has a range of legal
values, as well as a “zero” value that may be used
when you specify an illegal value that MySQL cannot represent. The
TIMESTAMP type has special automatic updating
behavior, described later on. For temporary type storage
requirements, see Section 11.5, “Data Type Storage Requirements”.
Starting from MySQL 5.0.2, MySQL gives warnings or errors if you
try to insert an illegal date. By setting the SQL mode to the
appropriate value, you can specify more exactly what kind of dates
you want MySQL to support. (See
Section 5.2.5, “The Server SQL Mode”.) You can get MySQL to accept
certain dates, such as '1999-11-31', by using
the ALLOW_INVALID_DATES SQL mode. (Before
5.0.2, this mode was the default behavior for MySQL.) This is
useful when you want to store a “possibly wrong”
value which the user has specified (for example, in a web form) in
the database for future processing. Under this mode, MySQL
verifies only that the month is in the range from 0 to 12 and that
the day is in the range from 0 to 31. These ranges are defined to
include zero because MySQL allows you to store dates where the day
or month and day are zero in a DATE or
DATETIME column. This is extremely useful for
applications that need to store a birthdate for which you do not
know the exact date. In this case, you simply store the date as
'1999-00-00' or
'1999-01-00'. If you store dates such as these,
you should not expect to get correct results for functions such as
DATE_SUB() or DATE_ADD that
require complete dates. (If you do not want
to allow zero in dates, you can use the
NO_ZERO_IN_DATE SQL mode).
MySQL also allows you to store '0000-00-00' as
a “dummy date” (if you are not using the
NO_ZERO_DATE SQL mode). This is in some cases
is more convenient (and uses less space in data and index) than
using NULL values.
Here are some general considerations to keep in mind when working with date and time types:
MySQL retrieves values for a given date or time type in a standard output format, but it attempts to interpret a variety of formats for input values that you supply (for example, when you specify a value to be assigned to or compared to a date or time type). Only the formats described in the following sections are supported. It is expected that you supply legal values. Unpredictable results may occur if you use values in other formats.
Dates containing two-digit year values are ambiguous because the century is unknown. MySQL interprets two-digit year values using the following rules:
Year values in the range 70-99 are
converted to 1970-1999.
Year values in the range 00-69 are
converted to 2000-2069.
Although MySQL tries to interpret values in several formats,
dates always must be given in year-month-day order (for
example, '98-09-04'), rather than in the
month-day-year or day-month-year orders commonly used
elsewhere (for example, '09-04-98',
'04-09-98').
MySQL automatically converts a date or time type value to a number if the value is used in a numeric context and vice versa.
By default, when MySQL encounters a value for a date or time
type that is out of range or otherwise illegal for the type
(as described at the beginning of this section), it converts
the value to the “zero” value for that type. The
exception is that out-of-range TIME values
are clipped to the appropriate endpoint of the
TIME range.
The following table shows the format of the
“zero” value for each type. Note that the use of
these values produces warnings if the
NO_ZERO_DATE SQL mode is enabled.
| Data Type | “Zero” Value |
DATETIME | '0000-00-00 00:00:00' |
DATE | '0000-00-00' |
TIMESTAMP | '0000-00-00 00:00:00' |
TIME | '00:00:00' |
YEAR | 0000 |
The “zero” values are special, but you can store
or refer to them explicitly using the values shown in the
table. You can also do this using the values
'0' or 0, which are
easier to write.
“Zero” date or time values used through MyODBC
are converted automatically to NULL in
MyODBC 2.50.12 and above, because ODBC cannot handle such
values.
The DATETIME, DATE, and
TIMESTAMP types are related. This section
describes their characteristics, how they are similar, and how
they differ.
The DATETIME type is used when you need
values that contain both date and time information. MySQL
retrieves and displays DATETIME values in
'YYYY-MM-DD HH:MM:SS' format. The supported
range is '1000-01-01 00:00:00' to
'9999-12-31 23:59:59'.
The DATE type is used when you need only a
date value, without a time part. MySQL retrieves and displays
DATE values in
'YYYY-MM-DD' format. The supported range is
'1000-01-01' to
'9999-12-31'.
For the DATETIME and DATE
range descriptions, “supported” means that although
earlier values might work, there is no guarantee.
The TIMESTAMP data type has varying
properties, depending on the MySQL version and the SQL mode the
server is running in. These properties are described later in
this section.
You can specify DATETIME,
DATE, and TIMESTAMP values
using any of a common set of formats:
As a string in either 'YYYY-MM-DD
HH:MM:SS' or 'YY-MM-DD
HH:MM:SS' format. A “relaxed” syntax
is allowed: Any punctuation character may be used as the
delimiter between date parts or time parts. For example,
'98-12-31 11:30:45', '98.12.31
11+30+45', '98/12/31 11*30*45',
and '98@12@31 11^30^45' are equivalent.
As a string in either 'YYYY-MM-DD' or
'YY-MM-DD' format. A
“relaxed” syntax is allowed here, too. For
example, '98-12-31',
'98.12.31',
'98/12/31', and
'98@12@31' are equivalent.
As a string with no delimiters in either
'YYYYMMDDHHMMSS' or
'YYMMDDHHMMSS' format, provided that the
string makes sense as a date. For example,
'19970523091528' and
'970523091528' are interpreted as
'1997-05-23 09:15:28', but
'971122129015' is illegal (it has a
nonsensical minute part) and becomes '0000-00-00
00:00:00'.
As a string with no delimiters in either
'YYYYMMDD' or 'YYMMDD'
format, provided that the string makes sense as a date. For
example, '19970523' and
'970523' are interpreted as
'1997-05-23', but
'971332' is illegal (it has nonsensical
month and day parts) and becomes
'0000-00-00'.
As a number in either YYYYMMDDHHMMSS or
YYMMDDHHMMSS format, provided that the
number makes sense as a date. For example,
19830905132800 and
830905132800 are interpreted as
'1983-09-05 13:28:00'.
As a number in either YYYYMMDD or
YYMMDD format, provided that the number
makes sense as a date. For example,
19830905 and 830905
are interpreted as '1983-09-05'.
As the result of a function that returns a value that is
acceptable in a DATETIME,
DATE, or TIMESTAMP
context, such as NOW() or
CURRENT_DATE.
Illegal DATETIME, DATE, or
TIMESTAMP values are converted to the
“zero” value of the appropriate type
('0000-00-00 00:00:00' or
'0000-00-00').
For values specified as strings that include date part
delimiters, it is not necessary to specify two digits for month
or day values that are less than 10.
'1979-6-9' is the same as
'1979-06-09'. Similarly, for values specified
as strings that include time part delimiters, it is not
necessary to specify two digits for hour, minute, or second
values that are less than 10.
'1979-10-30 1:2:3' is the same as
'1979-10-30 01:02:03'.
Values specified as numbers should be 6, 8, 12, or 14 digits
long. If a number is 8 or 14 digits long, it is assumed to be in
YYYYMMDD or YYYYMMDDHHMMSS
format and that the year is given by the first 4 digits. If the
number is 6 or 12 digits long, it is assumed to be in
YYMMDD or YYMMDDHHMMSS
format and that the year is given by the first 2 digits. Numbers
that are not one of these lengths are interpreted as though
padded with leading zeroes to the closest length.
Values specified as non-delimited strings are interpreted using
their length as given. If the string is 8 or 14 characters long,
the year is assumed to be given by the first 4 characters.
Otherwise, the year is assumed to be given by the first 2
characters. The string is interpreted from left to right to find
year, month, day, hour, minute, and second values, for as many
parts as are present in the string. This means you should not
use strings that have fewer than 6 characters. For example, if
you specify '9903', thinking that represents
March, 1999, MySQL inserts a “zero” date value into
your table. This occurs because the year and month values are
99 and 03, but the day
part is completely missing, so the value is not a legal date.
However, you can explicitly specify a value of zero to represent
missing month or day parts. For example, you can use
'990300' to insert the value
'1999-03-00'.
You can to some extent assign values of one date type to an object of a different date type. However, there may be some alteration of the value or loss of information:
If you assign a DATE value to a
DATETIME or TIMESTAMP
object, the time part of the resulting value is set to
'00:00:00' because the
DATE value contains no time information.
If you assign a DATETIME or
TIMESTAMP value to a
DATE object, the time part of the
resulting value is deleted because the
DATE type stores no time information.
Remember that although DATETIME,
DATE, and TIMESTAMP
values all can be specified using the same set of formats,
the types do not all have the same range of values. For
example, TIMESTAMP values cannot be
earlier than 1970 or later than
2037. This means that a date such as
'1968-01-01', while legal as a
DATETIME or DATE
value, is not valid as a TIMESTAMP value
and is converted to 0.
Be aware of certain pitfalls when specifying date values:
The relaxed format allowed for values specified as strings
can be deceiving. For example, a value such as
'10:11:12' might look like a time value
because of the ‘:’ delimiter,
but if used in a date context is interpreted as the year
'2010-11-12'. The value
'10:45:15' is converted to
'0000-00-00' because
'45' is not a legal month.
As of 5.0.2, the server requires that month and day values
be legal, and not merely in the range 1 to 12 and 1 to 31,
respectively. With strict mode disabled, invalid dates such
as '2004-04-31' are converted to
'0000-00-00' and a warning is generated.
With strict mode enabled, invalid dates generate an error.
To allow such dates, enable
ALLOW_INVALID_DATES. See
Section 5.2.5, “The Server SQL Mode”, for more information.
Before MySQL 5.0.2, the MySQL server performs only basic
checking on the validity of a date: The ranges for year,
month, and day are 1000 to 9999, 00 to 12, and 00 to 31,
respectively. Any date containing parts not within these
ranges is subject to conversion to
'0000-00-00'. Please note that this still
allows you to store invalid dates such as
'2002-04-31'. To ensure that a date is
valid, you should perform a check in your application.
Dates containing two-digit year values are ambiguous because the century is unknown. MySQL interprets two-digit year values using the following rules:
Year values in the range 00-69 are
converted to 2000-2069.
Year values in the range 70-99 are
converted to 1970-1999.
Note: In older versions of
MySQL (prior to 4.1), the properties of the
TIMESTAMP data type differed significantly
in many ways from what is described in this section. If you
need to convert older TIMESTAMP data to
work with MySQL 5.0, be sure to see the
MySQL 3.23, 4.0, 4.1 Reference Manual for details.
TIMESTAMP columns are displayed in the same
format as DATETIME columns. In other words,
the display width is fixed at 19 characters, and the format is
YYYY-MM-DD HH:MM:SS.
The MySQL server can be also be run with the
MAXDB SQL mode enabled. When the server
runs with this mode enabled, TIMESTAMP is
identical with DATETIME. That is, if this
mode is enabled at the time that a table is created,
TIMESTAMP columns are created as
DATETIME columns. As a result, such columns
use DATETIME display format, have the same
range of values, and there is no automatic initialization or
updating to the current date and time.
To enable MAXDB mode, set the server SQL
mode to MAXDB at startup using the
--sql-mode=MAXDB server option or by setting
the global sql_mode variable at runtime:
mysql> SET GLOBAL sql_mode=MAXDB;
A client can cause the server to run in
MAXDB mode for its own connection as
follows:
mysql> SET SESSION sql_mode=MAXDB;
Note that the information in the following discussion applies
to TIMESTAMP columns only for tables not
created with MAXDB mode enabled, because
such columns are created as DATETIME
columns.
As of MySQL 5.0.2, MySQL does not accept timestamp values that
include a zero in the day or month column or values that are
not a valid date. The sole exception to this rule is the
special value '0000-00-00 00:00:00'.
You have considerable flexibility in determining when
automatic TIMESTAMP initialization and
updating occur and which column should have those behaviors:
For one TIMESTAMP column in a table,
you can assign the current timestamp as the default value
and the auto-update value. It is possible to have the
current timestamp be the default value for initializing
the column, for the auto-update value, or both. It is not
possible to have the current timestamp be the default
value for one column and the auto-update value for another
column.
You can specify which TIMESTAMP column
to automatically initialize or update to the current date
and time. This need not be the first
TIMESTAMP column.
The following rules govern initialization and updating of
TIMESTAMP columns:
If a DEFAULT value is specified for the
first TIMESTAMP column in a table, it
is not ignored. The default can be
CURRENT_TIMESTAMP or a constant date
and time value.
DEFAULT NULL is the same as
DEFAULT CURRENT_TIMESTAMP for the
first TIMESTAMP
column. For any other TIMESTAMP column,
DEFAULT NULL is treated as
DEFAULT 0.
Any single TIMESTAMP column in a table
can be used as the one that is initialized to the current
timestamp or updated automatically.
In a CREATE TABLE statement, the first
TIMESTAMP column can be declared in any
of the following ways:
With both DEFAULT CURRENT_TIMESTAMP
and ON UPDATE CURRENT_TIMESTAMP
clauses, the column has the current timestamp for its
default value, and is automatically updated.
With neither DEFAULT nor
ON UPDATE clauses, it is the same
as DEFAULT CURRENT_TIMESTAMP ON UPDATE
CURRENT_TIMESTAMP.
With a DEFAULT CURRENT_TIMESTAMP
clause and no ON UPDATE clause, the
column has the current timestamp for its default value
but is not automatically updated.
With no DEFAULT clause and with an
ON UPDATE CURRENT_TIMESTAMP clause,
the column has a default of 0 and is automatically
updated.
With a constant DEFAULT value, the
column has the given default. If the column has an
ON UPDATE CURRENT_TIMESTAMP clause,
it is automatically updated, otherwise not.
In other words, you can use the current timestamp for both
the initial value and the auto-update value, or either
one, or neither. (For example, you can specify ON
UPDATE to enable auto-update without also having
the column auto-initialized.)
Any of CURRENT_TIMESTAMP,
CURRENT_TIMESTAMP(), or
NOW() can be used in the
DEFAULT and ON
UPDATE clauses. They all mean “the current
timestamp.”
The order of the DEFAULT and
ON UPDATE attributes does not matter.
If both DEFAULT and ON
UPDATE are specified for a
TIMESTAMP column, either can precede
the other. For example, these statements are equivalent:
CREATE TABLE t (ts TIMESTAMP);
CREATE TABLE t (ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP
ON UPDATE CURRENT_TIMESTAMP);
CREATE TABLE t (ts TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
DEFAULT CURRENT_TIMESTAMP);
To specify automatic default or updating for a
TIMESTAMP column other than the first
one, you must suppress the automatic initialization and
update behaviors for the first
TIMESTAMP column by explicitly
assigning it a constant DEFAULT value
(for example, DEFAULT 0 or
DEFAULT '2003-01-01 00:00:00'). Then,
for the other TIMESTAMP column, the
rules are the same as for the first
TIMESTAMP column, except that if you
omit both of the DEFAULT and
ON UPDATE clauses, no automatic
initialization or updating occurs.
Example. These statements are equivalent:
CREATE TABLE t (
ts1 TIMESTAMP DEFAULT 0,
ts2 TIMESTAMP DEFAULT CURRENT_TIMESTAMP
ON UPDATE CURRENT_TIMESTAMP);
CREATE TABLE t (
ts1 TIMESTAMP DEFAULT 0,
ts2 TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
DEFAULT CURRENT_TIMESTAMP);
You can set the current time zone on a per-connection basis,
as described in Section 5.11.8, “MySQL Server Time Zone Support”.
TIMESTAMP values are stored in UTC, being
converted from the current time zone for storage, and
converted back to the current time zone upon retrieval. As
long as the time zone setting remains constant, you get back
the same value you store. If you store a
TIMESTAMP value, and then change the time
zone and retrieve the value, the retrieved value is different
than the value you stored. This occurs because the same time
zone was not used for conversion in both directions. The
current time zone is available as the value of the
time_zone system variable.
You can include the NULL attribute in the
definition of a TIMESTAMP column to allow
the column to contain NULL values. For
example:
CREATE TABLE t ( ts1 TIMESTAMP NULL DEFAULT NULL, ts2 TIMESTAMP NULL DEFAULT 0, ts3 TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP );
If the NULL attribute is not specified,
setting the column to NULL sets it to the
current timestamp. Note that a TIMESTAMP
column which allows NULL values will
not take on the current timestamp except
under one of the following conditions:
Its default value is defined as
CURRENT_TIMESTAMP
NOW() or
CURRENT_TIMESTAMP is inserted into the
column
In other words, a TIMESTAMP column defined
as NULL will auto-initialize only if it is
created using a definition such as the following:
CREATE TABLE t (ts TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP);
Otherwise — that is, if the TIMESTAMP
column is defined to allow NULL values but
not using DEFAULT TIMESTAMP, as shown
here…
CREATE TABLE t1 (ts TIMESTAMP NULL DEFAULT NULL); CREATE TABLE t2 (ts TIMESTAMP NULL DEFAULT '0000-00-00 00:00:00');
…then you must explicitly insert a value corresponding to the current date and time. For example:
INSERT INTO t1 VALUES (NOW()); INSERT INTO t2 VALUES (CURRENT_TIMESTAMP);
MySQL retrieves and displays TIME values in
'HH:MM:SS' format (or
'HHH:MM:SS' format for large hours values).
TIME values may range from
'-838:59:59' to
'838:59:59'. The hours part may be so large
because the TIME type can be used not only to
represent a time of day (which must be less than 24 hours), but
also elapsed time or a time interval between two events (which
may be much greater than 24 hours, or even negative).
You can specify TIME values in a variety of
formats:
As a string in 'D HH:MM:SS.fraction'
format. You can also use one of the following
“relaxed” syntaxes:
'HH:MM:SS.fraction',
'HH:MM:SS', 'HH:MM',
'D HH:MM:SS', 'D
HH:MM', 'D HH', or
'SS'. Here D
represents days and can have a value from 0 to 34. Note that
MySQL does not store the fraction part.
As a string with no delimiters in
'HHMMSS' format, provided that it makes
sense as a time. For example, '101112' is
understood as '10:11:12', but
'109712' is illegal (it has a nonsensical
minute part) and becomes '00:00:00'.
As a number in HHMMSS format, provided
that it makes sense as a time. For example,
101112 is understood as
'10:11:12'. The following alternative
formats are also understood: SS,
MMSS, HHMMSS,
HHMMSS.fraction. Note that MySQL does not
store the fraction part.
As the result of a function that returns a value that is
acceptable in a TIME context, such as
CURRENT_TIME.
For TIME values specified as strings that
include a time part delimiter, it is not necessary to specify
two digits for hours, minutes, or seconds values that are less
than 10. '8:3:2' is the
same as '08:03:02'.
Be careful about assigning abbreviated values to a
TIME column. Without colons, MySQL interprets
values using the assumption that the two rightmost digits
represent seconds. (MySQL interprets TIME
values as elapsed time rather than as time of day.) For example,
you might think of '1112' and
1112 as meaning '11:12:00'
(12 minutes after 11 o'clock), but MySQL interprets them as
'00:11:12' (11 minutes, 12 seconds).
Similarly, '12' and 12 are
interpreted as '00:00:12'.
TIME values with colons, by contrast, are
always treated as time of the day. That is,
'11:12' mean '11:12:00',
not '00:11:12'.
By default, values that lie outside the TIME
range but are otherwise legal are clipped to the closest
endpoint of the range. For example,
'-850:00:00' and
'850:00:00' are converted to
'-838:59:59' and
'838:59:59'. Illegal TIME
values are converted to '00:00:00'. Note that
because '00:00:00' is itself a legal
TIME value, there is no way to tell, from a
value of '00:00:00' stored in a table,
whether the original value was specified as
'00:00:00' or whether it was illegal.
For more restrictive treatment of invalid
TIME values, enable strict SQL mode to cause
errors to occur. See Section 5.2.5, “The Server SQL Mode”.
The YEAR type is a one-byte type used for
representing years.
MySQL retrieves and displays YEAR values in
YYYY format. The range is
1901 to 2155.
You can specify YEAR values in a variety of
formats:
As a four-digit string in the range
'1901' to '2155'.
As a four-digit number in the range 1901
to 2155.
As a two-digit string in the range '00'
to '99'. Values in the ranges
'00' to '69' and
'70' to '99' are
converted to YEAR values in the ranges
2000 to 2069 and
1970 to 1999.
As a two-digit number in the range 1 to
99. Values in the ranges
1 to 69 and
70 to 99 are converted
to YEAR values in the ranges
2001 to 2069 and
1970 to 1999. Note
that the range for two-digit numbers is slightly different
from the range for two-digit strings, because you cannot
specify zero directly as a number and have it be interpreted
as 2000. You must specify it as a string
'0' or '00' or it is
interpreted as 0000.
As the result of a function that returns a value that is
acceptable in a YEAR context, such as
NOW().
Illegal YEAR values are converted to
0000.
As discussed in Section 1.4.5, “Year 2000 Compliance”, MySQL itself is year 2000 (Y2K) safe. However, particular input values presented to MySQL may not be Y2K safe. Any value containing a two-digit year is ambiguous, because the century is unknown. Such values must be interpreted into four-digit form because MySQL stores years internally using four digits.
For DATETIME, DATE,
TIMESTAMP, and YEAR types,
MySQL interprets dates with ambiguous year values using the
following rules:
Year values in the range 00-69 are
converted to 2000-2069.
Year values in the range 70-99 are
converted to 1970-1999.
Remember that these rules are only heuristics that provide reasonable guesses as to what your data values mean. If the rules used by MySQL do not produce the correct values, you should provide unambiguous input containing four-digit year values.
ORDER BY properly sorts
YEAR values that have two-digit years.
Some functions like MIN() and
MAX() convert a YEAR to a
number. This means that a value with a two-digit year does not
work properly with these functions. The fix in this case is to
convert the TIMESTAMP or
YEAR to four-digit year format.
The string types are CHAR,
VARCHAR, BINARY,
VARBINARY, BLOB,
TEXT, ENUM, and
SET. This section describes how these types
work and how to use them in your queries. For string type storage
requirements, see Section 11.5, “Data Type Storage Requirements”.
The CHAR and VARCHAR types
are similar, but differ in the way they are stored and
retrieved. As of MySQL 5.0.3, they also differ in maximum length
and in whether trailing spaces are retained.
The CHAR and VARCHAR types
are declared with a length that indicates the maximum number of
characters you want to store. For example,
CHAR(30) can hold up to 30 characters.
The length of a CHAR column is fixed to the
length that you declare when you create the table. The length
can be any value from 0 to 255. When CHAR
values are stored, they are right-padded with spaces to the
specified length. When CHAR values are
retrieved, trailing spaces are removed.
Values in VARCHAR columns are variable-length
strings. The length can be specified as a value from 0 to 255
before MySQL 5.0.3, and 0 to 65,535 in 5.0.3 and later versions.
(The maximum effective length of a VARCHAR in
MySQL 5.0.3 and later is determined by the maximum row size and
the character set used. The maximum length overall is 65,532
bytes.)
In contrast to CHAR,
VARCHAR values are stored using only as many
characters as are needed, plus one byte to record the length
(two bytes for columns that are declared with a length longer
than 255).
VARCHAR values are not padded when they are
stored. Handling of trailing spaces is version-dependent. As of
MySQL 5.0.3, trailing spaces are retained when values are stored
and retrieved, in conformance with standard SQL. Before MySQL
5.0.3, trailing spaces are removed from values when they are
stored into a VARCHAR column; this means that
the spaces also are absent from retrieved values.
If you assign a value to a CHAR or
VARCHAR column that exceeds the column's
maximum length, the value is truncated to fit. If the truncated
characters are not spaces, a warning is generated. For
truncation of non-space characters, you can cause an error to
occur (rather than a warning) and suppress insertion of the
value by using strict SQL mode. See
Section 5.2.5, “The Server SQL Mode”.
Before MySQL 5.0.3, if you need a data type for which trailing
spaces are not removed, consider using a BLOB
or TEXT type. Also, if you want to store
binary values such as results from an encryption or compression
function that might contain arbitrary byte values, use a
BLOB column rather than a
CHAR or VARCHAR column, to
avoid potential problems with trailing space removal that would
change data values.
The following table illustrates the differences between
CHAR and VARCHAR by
showing the result of storing various string values into
CHAR(4) and VARCHAR(4)
columns:
| Value | CHAR(4) | Storage Required | VARCHAR(4) | Storage Required |
'' | ' ' | 4 bytes | '' | 1 byte |
'ab' | 'ab ' | 4 bytes | 'ab' | 3 bytes |
'abcd' | 'abcd' | 4 bytes | 'abcd' | 5 bytes |
'abcdefgh' | 'abcd' | 4 bytes | 'abcd' | 5 bytes |
Note that the values shown as stored in the last row of the table apply only when not using strict mode; if MySQL is running in strict mode, values that exceed the column length are not stored, and an error results.
If a given value is stored into the CHAR(4)
and VARCHAR(4) columns, the values retrieved
from the columns are not always the same because trailing spaces
are removed from CHAR columns upon retrieval.
The following example illustrates this difference:
mysql>CREATE TABLE vc (v VARCHAR(4), c CHAR(4));Query OK, 0 rows affected (0.01 sec) mysql>INSERT INTO vc VALUES ('ab ', 'ab ');Query OK, 1 row affected (0.00 sec) mysql>SELECT CONCAT('(', v, ')'), CONCAT('(', c, ')') FROM vc;+---------------------+---------------------+ | CONCAT('(', v, ')') | CONCAT('(', c, ')') | +---------------------+---------------------+ | (ab ) | (ab) | +---------------------+---------------------+ 1 row in set (0.06 sec)
Values in CHAR and VARCHAR
columns are sorted and compared according to the character set
collation assigned to the column.
Note that all MySQL collations are of type
PADSPACE. This means that all
CHAR and VARCHAR values in
MySQL are compared without regard to any trailing spaces. For
example:
mysql>CREATE TABLE names (myname CHAR(10), yourname VARCHAR(10));Query OK, 0 rows affected (0.09 sec) mysql>INSERT INTO names VALUES ('Monty ', 'Monty ');Query OK, 1 row affected (0.00 sec) mysql>SELECT myname = 'Monty ', yourname = 'Monty ' FROM names;+--------------------+----------------------+ | myname = 'Monty ' | yourname = 'Monty ' | +--------------------+----------------------+ | 1 | 1 | +--------------------+----------------------+ 1 row in set (0.00 sec)
Note that this is true for all MySQL versions, and it makes no
difference whether your version trims trailing spaces from
VARCHAR values before storing them. Nor does
the server SQL mode make any difference in this regard.
For those cases where trailing pad characters are stripped or
comparisons ignore them, if a column has an index that requires
unique values, inserting into the column values that differ only
in number of trailing pad characters will result in a
duplicate-key error. For example, if a table contains
'a', an attempt to store
'a ' causes a duplicate-key error.
The BINARY and VARBINARY
types are similar to CHAR and
VARCHAR, except that they contain binary
strings rather than non-binary strings. That is, they contain
byte strings rather than character strings. This means that they
have no character set, and sorting and comparison are based on
the numeric values of the bytes in the values.
The allowable maximum length is the same for
BINARY and VARBINARY as it
is for CHAR and VARCHAR,
except that the length for BINARY and
VARBINARY is a length in bytes rather than in
characters.
The BINARY and VARBINARY
data types are distinct from the CHAR BINARY
and VARCHAR BINARY data types. For the latter
types, the BINARY attribute does not cause
the column to be treated as a binary string column. Instead, it
causes the binary collation for the column character set to be
used, and the column itself contains non-binary character
strings rather than binary byte strings. For example,
CHAR(5) BINARY is treated as CHAR(5)
CHARACTER SET latin1 COLLATE latin1_bin, assuming that
the default character set is latin1. This
differs from BINARY(5), which stores 5-bytes
binary strings that have no character set or collation.
When BINARY values are stored, they are
right-padded with the pad value to the specified length. The pad
value and how it is handled is version specific:
As of MySQL 5.0.15, the pad value is 0x00
(the zero byte). Values are right-padded with
0x00 on insert, and no trailing bytes are
removed on select. All bytes are significant in comparisons,
including ORDER BY and
DISTINCT operations.
0x00 bytes and spaces are different in
comparisons, with 0x00 < space.
Example: For a BINARY(3) column,
'a ' becomes
'a \0' when inserted.
'a\0' becomes 'a\0\0'
when inserted. Both inserted values remain unchanged when
selected.
Before MySQL 5.0.15, the pad value is space. Values are
right-padded with space on insert, and trailing spaces are
removed on select. Trailing spaces are ignored in
comparisons, including ORDER BY and
DISTINCT operations.
0x00 bytes and spaces are different in
comparisons, with 0x00 < space.
Example: For a BINARY(3) column,
'a ' becomes
'a ' when inserted and
'a' when selected.
'a\0' becomes
'a\0 ' when inserted and
'a\0' when selected.
For VARBINARY, there is no padding on insert
and no bytes are stripped on select. All bytes are significant
in comparisons, including ORDER BY and
DISTINCT operations. 0x00
bytes and spaces are different in comparisons, with
0x00 < space. (Exceptions: Before MySQL
5.0.3, trailing spaces are removed when values are stored.
Before MySQL 5.0.15, trailing 0x00 bytes are removed for
ORDER BY operations.)
Note: The InnoDB storage engine continues to
preserve trailing spaces in BINARY and
VARBINARY column values through MySQL 5.0.18.
Beginning with MySQL 5.0.19, InnoDB uses
trailing space characters in making comparisons as do other
MySQL storage engines.
For those cases where trailing pad bytes are stripped or
comparisons ignore them, if a column has an index that requires
unique values, inserting into the column values that differ only
in number of trailing pad bytes will result in a duplicate-key
error. For example, if a table contains 'a',
an attempt to store 'a\0' causes a
duplicate-key error.
You should consider the preceding padding and stripping
characteristics carefully if you plan to use the
BINARY data type for storing binary data and
you require that the value retrieved be exactly the same as the
value stored. The following example illustrates how
0x00-padding of BINARY
values affects column value comparisons:
mysql>CREATE TABLE t (c BINARY(3));Query OK, 0 rows affected (0.01 sec) mysql>INSERT INTO t SET c = 'a';Query OK, 1 row affected (0.01 sec) mysql>SELECT HEX(c), c = 'a', c = 'a\0\0' from t;+--------+---------+-------------+ | HEX(c) | c = 'a' | c = 'a\0\0' | +--------+---------+-------------+ | 610000 | 0 | 1 | +--------+---------+-------------+ 1 row in set (0.09 sec)
If the value retrieved must be the same as the value specified
for storage with no padding, it might be preferable to use
VARBINARY or one of the
BLOB data types instead.
A BLOB is a binary large object that can hold
a variable amount of data. The four BLOB
types are TINYBLOB, BLOB,
MEDIUMBLOB, and LONGBLOB.
These differ only in the maximum length of the values they can
hold. The four TEXT types are
TINYTEXT, TEXT,
MEDIUMTEXT, and LONGTEXT.
These correspond to the four BLOB types and
have the same maximum lengths and storage requirements. See
Section 11.5, “Data Type Storage Requirements”. No lettercase conversion
for TEXT or BLOB columns
takes place during storage or retrieval.
BLOB columns are treated as binary strings
(byte strings). TEXT columns are treated as
non-binary strings (character strings). BLOB
columns have no character set, and sorting and comparison are
based on the numeric values of the bytes in column values.
TEXT columns have a character set, and values
are sorted and compared based on the collation of the character
set.
If a TEXT column is indexed, index entry
comparisons are space-padded at the end. This means that, if the
index requires unique values, duplicate-key errors will occur
for values that differ only in the number of trailing spaces.
For example, if a table contains 'a', an
attempt to store 'a ' causes a
duplicate-key error. This is not true for
BLOB columns.
When not running in strict mode, if you assign a value to a
BLOB or TEXT column that
exceeds the data type's maximum length, the value is truncated
to fit. If the truncated characters are not spaces, a warning is
generated. You can cause an error to occur and the value to be
rejected rather than to be truncated with a warning by using
strict SQL mode. See Section 5.2.5, “The Server SQL Mode”.
In most respects, you can regard a BLOB
column as a VARBINARY column that can be as
large as you like. Similarly, you can regard a
TEXT column as a VARCHAR
column. BLOB and TEXT
differ from VARBINARY and
VARCHAR in the following ways:
There is no trailing-space removal for
BLOB and TEXT columns
when values are stored or retrieved. Before MySQL 5.0.3,
this differs from VARBINARY and
VARCHAR, for which trailing spaces are
removed when values are stored.
Note that TEXT is on comparison space
extended to fit the compared object, exactly like
CHAR and VARCHAR.
For indexes on BLOB and
TEXT columns, you must specify an index
prefix length. For CHAR and
VARCHAR, a prefix length is optional. See
Section 7.4.3, “Column Indexes”.
LONG and LONG VARCHAR map
to the MEDIUMTEXT data type. This is a
compatibility feature. If you use the BINARY
attribute with a TEXT data type, the column
is assigned the binary collation of the column character set.
MySQL Connector/ODBC defines BLOB values as
LONGVARBINARY and TEXT
values as LONGVARCHAR.
Because BLOB and TEXT
values can be extremely long, you might encounter some
constraints in using them:
Only the first max_sort_length bytes of
the column are used when sorting. The default value of
max_sort_length is 1024. This value can
be changed using the
--max_sort_length=
option when starting the mysqld server.
See Section 5.2.2, “Server System Variables”.
N
You can make more bytes significant in sorting or grouping
by increasing the value of
max_sort_length at runtime. Any client
can change the value of its session
max_sort_length variable:
mysql>SET max_sort_length = 2000;mysql>SELECT id, comment FROM t->ORDER BY comment;
Another way to use GROUP BY or
ORDER BY on a BLOB or
TEXT column containing long values when
you want more than max_sort_length bytes
to be significant is to convert the column value into a
fixed-length object. The standard way to do this is with the
SUBSTRING() function. For example, the
following statement causes 2000 bytes of the
comment column to be taken into account
for sorting:
mysql>SELECT id, SUBSTRING(comment,1,2000) FROM t->ORDER BY SUBSTRING(comment,1,2000);
The maximum size of a BLOB or
TEXT object is determined by its type,
but the largest value you actually can transmit between the
client and server is determined by the amount of available
memory and the size of the communications buffers. You can
change the message buffer size by changing the value of the
max_allowed_packet variable, but you must
do so for both the server and your client program. For
example, both mysql and
mysqldump allow you to change the
client-side max_allowed_packet value. See
Section 7.5.2, “Tuning Server Parameters”,
Section 8.5, “mysql — The MySQL Command-Line Tool”, and Section 8.10, “mysqldump — A Database Backup Program”.
Each BLOB or TEXT value is
represented internally by a separately allocated object. This is
in contrast to all other data types, for which storage is
allocated once per column when the table is opened.
In some cases, it may be desirable to store binary data such as
media files in BLOB or
TEXT columns. You may find MySQL's string
handling functions useful for working with such data. See
Section 12.3, “String Functions”. For security and other
reasons, it is usually preferable to do so using application
code rather than allowing application users the
FILE privilege. You can discuss specifics for
various languages and platforms in the MySQL Forums
(http://forums.mysql.com/).
An ENUM is a string object with a value
chosen from a list of allowed values that are enumerated
explicitly in the column specification at table creation time.
The value may also be the empty string ('')
or NULL under certain circumstances:
If you insert an invalid value into an
ENUM (that is, a string not present in
the list of allowed values), the empty string is inserted
instead as a special error value. This string can be
distinguished from a “normal” empty string by
the fact that this string has the numerical value 0. More
about this later.
If strict SQL mode is enabled, attempts to insert invalid
ENUM values result in an error.
If an ENUM column is declared to allow
NULL, the NULL value
is a legal value for the column, and the default value is
NULL. If an ENUM
column is declared NOT NULL, its default
value is the first element of the list of allowed values.
Each enumeration value has an index:
Values from the list of allowable elements in the column specification are numbered beginning with 1.
The index value of the empty string error value is 0. This
means that you can use the following
SELECT statement to find rows into which
invalid ENUM values were assigned:
mysql> SELECT * FROM tbl_name WHERE enum_col=0;
The index of the NULL value is
NULL.
The term “index” here refers only to position within the list of enumeration values. It has nothing to do with table indexes.
For example, a column specified as ENUM('one', 'two',
'three') can have any of the values shown here. The
index of each value is also shown:
| Value | Index |
NULL | NULL |
'' | 0 |
'one' | 1 |
'two' | 2 |
'three' | 3 |
An enumeration can have a maximum of 65,535 elements.
Trailing spaces are automatically deleted from
ENUM member values in the table definition
when a table is created.
When retrieved, values stored into an ENUM
column are displayed using the lettercase that was used in the
column definition. Note that ENUM columns can
be assigned a character set and collation. For binary or
case-sensitive collations, lettercase is taken into account when
assigning values to the column.
If you retrieve an ENUM value in a numeric
context, the column value's index is returned. For example, you
can retrieve numeric values from an ENUM
column like this:
mysql> SELECT enum_col+0 FROM tbl_name;
If you store a number into an ENUM column,
the number is treated as an index, and the value stored is the
enumeration member with that index. (However, this does not work
with LOAD DATA, which treats all input as
strings.) It is not advisable to define an
ENUM column with enumeration values that look
like numbers, because this can easily become confusing. For
example, the following column has enumeration members with
string values of '0', '1',
and '2', but numeric index values of
1, 2, and
3:
numbers ENUM('0','1','2')
ENUM values are sorted according to the order
in which the enumeration members were listed in the column
specification. (In other words, ENUM values
are sorted according to their index numbers.) For example,
'a' sorts before 'b' for
ENUM('a', 'b'), but 'b'
sorts before 'a' for ENUM('b',
'a'). The empty string sorts before non-empty strings,
and NULL values sort before all other
enumeration values. To prevent unexpected results, specify the
ENUM list in alphabetical order. You can also
use GROUP BY CAST(col AS CHAR) or
GROUP BY CONCAT(col) to make sure that the
column is sorted lexically rather than by index number.
If you want to determine all possible values for an
ENUM column, use SHOW COLUMNS FROM
and parse the
tbl_name LIKE
enum_colENUM definition in the
Type column of the output.
A SET is a string object that can have zero
or more values, each of which must be chosen from a list of
allowed values specified when the table is created.
SET column values that consist of multiple
set members are specified with members separated by commas
(‘,’). A consequence of this is
that SET member values should not themselves
contain commas.
For example, a column specified as SET('one', 'two')
NOT NULL can have any of these values:
'' 'one' 'two' 'one,two'
A SET can have a maximum of 64 different
members.
Trailing spaces are automatically deleted from
SET member values in the table definition
when a table is created.
When retrieved, values stored in a SET column
are displayed using the lettercase that was used in the column
definition. Note that SET columns can be
assigned a character set and collation. For binary or
case-sensitive collations, lettercase is taken into account when
assigning values to the column.
MySQL stores SET values numerically, with the
low-order bit of the stored value corresponding to the first set
member. If you retrieve a SET value in a
numeric context, the value retrieved has bits set corresponding
to the set members that make up the column value. For example,
you can retrieve numeric values from a SET
column like this:
mysql> SELECT set_col+0 FROM tbl_name;
If a number is stored into a SET column, the
bits that are set in the binary representation of the number
determine the set members in the column value. For a column
specified as SET('a','b','c','d'), the
members have the following decimal and binary values:
SET Member | Decimal Value | Binary Value |
'a' | 1 | 0001 |
'b' | 2 | 0010 |
'c' | 4 | 0100 |
'd' | 8 | 1000 |
If you assign a value of 9 to this column,
that is 1001 in binary, so the first and
fourth SET value members
'a' and 'd' are selected
and the resulting value is 'a,d'.
For a value containing more than one SET
element, it does not matter what order the elements are listed
in when you insert the value. It also does not matter how many
times a given element is listed in the value. When the value is
retrieved later, each element in the value appears once, with
elements listed according to the order in which they were
specified at table creation time. For example, suppose that a
column is specified as SET('a','b','c','d'):
mysql> CREATE TABLE myset (col SET('a', 'b', 'c', 'd'));
If you insert the values 'a,d',
'd,a', 'a,d,d',
'a,d,a', and 'd,a,d':
mysql> INSERT INTO myset (col) VALUES
-> ('a,d'), ('d,a'), ('a,d,a'), ('a,d,d'), ('d,a,d');
Query OK, 5 rows affected (0.01 sec)
Records: 5 Duplicates: 0 Warnings: 0
Then all of these values appear as 'a,d' when
retrieved:
mysql> SELECT col FROM myset;
+------+
| col |
+------+
| a,d |
| a,d |
| a,d |
| a,d |
| a,d |
+------+
5 rows in set (0.04 sec)
If you set a SET column to an unsupported
value, the value is ignored and a warning is issued:
mysql>INSERT INTO myset (col) VALUES ('a,d,d,s');Query OK, 1 row affected, 1 warning (0.03 sec) mysql>SHOW WARNINGS;+---------+------+------------------------------------------+ | Level | Code | Message | +---------+------+------------------------------------------+ | Warning | 1265 | Data truncated for column 'col' at row 1 | +---------+------+------------------------------------------+ 1 row in set (0.04 sec) mysql>SELECT col FROM myset;+------+ | col | +------+ | a,d | | a,d | | a,d | | a,d | | a,d | | a,d | +------+ 6 rows in set (0.01 sec)
If strict SQL mode is enabled, attempts to insert invalid
SET values result in an error.
SET values are sorted numerically.
NULL values sort before
non-NULL SET values.
Normally, you search for SET values using the
FIND_IN_SET() function or the
LIKE operator:
mysql>SELECT * FROMmysql>tbl_nameWHERE FIND_IN_SET('value',set_col)>0;SELECT * FROMtbl_nameWHEREset_colLIKE '%value%';
The first statement finds rows where
set_col contains the
value set member. The second is
similar, but not the same: It finds rows where
set_col contains
value anywhere, even as a substring
of another set member.
The following statements also are legal:
mysql>SELECT * FROMmysql>tbl_nameWHEREset_col& 1;SELECT * FROMtbl_nameWHEREset_col= 'val1,val2';
The first of these statements looks for values containing the
first set member. The second looks for an exact match. Be
careful with comparisons of the second type. Comparing set
values to
'
returns different results than comparing values to
val1,val2''.
You should specify the values in the same order they are listed
in the column definition.
val2,val1'
If you want to determine all possible values for a
SET column, use SHOW COLUMNS FROM
and parse the
tbl_name LIKE
set_colSET definition in the Type
column of the output.
The storage requirements for each of the data types supported by MySQL are listed here by category.
The maximum size of a row in a MyISAM table is
65,534 bytes. Each BLOB and
TEXT column accounts for only five to nine
bytes toward this size.
Storage Requirements for Numeric Types
| Data Type | Storage Required |
TINYINT | 1 byte |
SMALLINT | 2 bytes |
MEDIUMINT | 3 bytes |
INT, INTEGER | 4 bytes |
BIGINT | 8 bytes |
FLOAT( | 4 bytes if 0 <= p <= 24, 8 bytes if 25
<= p <= 53 |
FLOAT | 4 bytes |
DOUBLE [PRECISION], REAL | 8 bytes |
DECIMAL(,
NUMERIC( | Varies; see following discussion |
BIT( | approximately (M+7)/8 bytes |
The storage requirements for DECIMAL (and
NUMERIC) are version-specific:
As of MySQL 5.0.3, values for DECIMAL columns
are represented using a binary format that packs nine decimal
(base 10) digits into four bytes. Storage for the integer and
fractional parts of each value are determined separately. Each
multiple of nine digits requires four bytes, and the
“leftover” digits require some fraction of four
bytes. The storage required for excess digits is given by the
following table:
| Leftover Digits | Number of Bytes |
| 0 | 0 |
| 1 | 1 |
| 2 | 1 |
| 3 | 2 |
| 4 | 2 |
| 5 | 3 |
| 6 | 3 |
| 7 | 4 |
| 8 | 4 |
| 9 | 4 |
Before MySQL 5.0.3, DECIMAL columns are
represented as strings and storage requirements are:
M+2 bytes if
D > 0,
M+1D = 0 (D+2,
if M <
D
Storage Requirements for Date and Time Types
| Data Type | Storage Required |
DATE | 3 bytes |
DATETIME | 8 bytes |
TIMESTAMP | 4 bytes |
TIME | 3 bytes |
YEAR | 1 byte |
Storage Requirements for String Types
| Data Type | Storage Required |
CHAR( | <=
255 |
VARCHAR( | Prior to MySQL 5.0.3: L
+ 1 bytes, where <= 255. MySQL 5.0.3 and
later: L + 1 bytes,
where <= 255 or
L + 2 bytes, where
<= 65535 (see note below). |
BINARY( | <=
255 |
VARBINARY( | Prior to MySQL 5.0.3: L
+ 1 bytes, where <= 255. MySQL 5.0.3 and
later: L + 1 bytes,
where <= 255 or
L + 2 bytes, where
<= 65535 (see note below). |
TINYBLOB, TINYTEXT | L+1 byte, where L
< 28 |
BLOB, TEXT | L+2 bytes, where L
< 216 |
MEDIUMBLOB, MEDIUMTEXT | L+3 bytes, where L
< 224 |
LONGBLOB, LONGTEXT | L+4 bytes, where L
< 232 |
ENUM(' | 1 or 2 bytes, depending on the number of enumeration values (65,535 values maximum) |
SET(' | 1, 2, 3, 4, or 8 bytes, depending on the number of set members (64 members maximum) |
For the CHAR, VARCHAR, and
TEXT types, the values
L and M in
the preceding table should be interpreted as number of characters,
and lengths for these types in column specifications indicate the
number of characters. For example, to store a
TINYTEXT value requires
L characters plus one byte.
VARCHAR, VARBINARY, and the
BLOB and TEXT types are
variable-length types. For each, the storage requirements depend
on these factors:
The actual length of the column value
The column's maximum possible length
The character set used for the column
For example, a VARCHAR(10) column can hold a
string with a maximum length of 10. Assuming that the column uses
the latin1 character set (one byte per
character), the actual storage required is the length of the
string (L), plus one byte to record the
length of the string. For the string 'abcd',
L is 4 and the storage requirement is
five bytes. If the same column was instead declared as
VARCHAR(500), the string
'abcd' requires 4 + 2 = 6 bytes. Two bytes
rather than one are required for the prefix because the length of
the column is greater than 255 characters.
To calculate the number of bytes used to
store a particular CHAR,
VARCHAR, or TEXT column
value, you must take into account the character set used for that
column. In particular, when using the utf8
Unicode character set, you must keep in mind that not all
utf8 characters use the same number of bytes.
For a breakdown of the storage used for different categories of
utf8 characters, see
Section 10.7, “Unicode Support”.
Note: In MySQL 5.0.3 and later,
the effective maximum length for a
VARCHAR or VARBINARY column
is 65,532.
As of MySQL 5.0.3, the NDBCLUSTER engine
supports only fixed-width columns. This means that a
VARCHAR column from a table in a MySQL Cluster
will behave as follows:
If the size of the column is fewer than 256 characters, the column requires one byte extra storage per row.
If the size of the column is 256 characters or more, the column requires two bytes extra storage per row.
Note that the number of bytes required per character varies
according to the character set used. For example, if a
VARCHAR(100) column in a Cluster table uses the
utf8 character set, then each character
requires 3 bytes storage. This means that each record in such a
column takes up 100 × 3 + 1 = 301 bytes for
storage, regardless of the length of the string actually stored in
any given record. For a VARCHAR(1000) column in
a table using the NDBCLUSTER storage engine
with the utf8 character set, each record will
use 1000 × 3 + 2 = 3002 bytes storage; that is,
the column is 1,000 characters wide, each character requires 3
bytes storage, and each record has a 2-byte overhead because 1,000
> 256.
The BLOB and TEXT types
require 1, 2, 3, or 4 bytes to record the length of the column
value, depending on the maximum possible length of the type. See
Section 11.4.3, “The BLOB and TEXT Types”.
TEXT and BLOB columns are
implemented differently in the NDB Cluster storage engine, wherein
each row in a TEXT column is made up of two
separate parts. One of these is of fixed size (256 bytes), and is
actually stored in the original table. The other consists of any
data in excess of 256 bytes, which stored in a hidden table. The
rows in this second table are always 2,000 bytes long. This means
that the size of a TEXT column is 256 if
size <= 256 (where
size represents the size of the row);
otherwise, the size is 256 + size +
(2000 – (size – 256) %
2000).
The size of an ENUM object is determined by the
number of different enumeration values. One byte is used for
enumerations with up to 255 possible values. Two bytes are used
for enumerations having between 256 and 65,535 possible values.
See Section 11.4.4, “The ENUM Type”.
The size of a SET object is determined by the
number of different set members. If the set size is
N, the object occupies
( bytes,
rounded up to 1, 2, 3, 4, or 8 bytes. A N+7)/8SET can
have a maximum of 64 members. See Section 11.4.5, “The SET Type”.
For optimum storage, you should try to use the most precise type
in all cases. For example, if an integer column is used for values
in the range from 1 to
99999, MEDIUMINT UNSIGNED is
the best type. Of the types that represent all the required
values, this type uses the least amount of storage.
Tables created in MySQL 5.0.3 and above uses a new storage format
for DECIMAL columns. All basic calculation
(+,-,*,/) with DECIMAL
columns are done with precision of 65 decimal (base 10) digits.
See Section 11.1.1, “Overview of Numeric Types”.
Prior to MySQL 5.0.3, calculations on DECIMAL
values are performed using double-precision operations. If
accuracy is not too important or if speed is the highest priority,
the DOUBLE type may be good enough. For high
precision, you can always convert to a fixed-point type stored in
a BIGINT. This allows you to do all
calculations with 64-bit integers and then convert results back to
floating-point values as necessary.
To facilitate the use of code written for SQL implementations from other vendors, MySQL maps data types as shown in the following table. These mappings make it easier to import table definitions from other database systems into MySQL:
| Other Vendor Type | MySQL Type |
BOOL, | TINYINT |
BOOLEAN | TINYINT |
CHAR VARYING( | VARCHAR( |
DEC | DECIMAL |
FIXED | DECIMAL |
FLOAT4 | FLOAT |
FLOAT8 | DOUBLE |
INT1 | TINYINT |
INT2 | SMALLINT |
INT3 | MEDIUMINT |
INT4 | INT |
INT8 | BIGINT |
LONG VARBINARY | MEDIUMBLOB |
LONG VARCHAR | MEDIUMTEXT |
LONG | MEDIUMTEXT |
MIDDLEINT | MEDIUMINT |
NUMERIC | DECIMAL |
Data type mapping occurs at table creation time, after which the
original type specifications are discarded. If you create a table
with types used by other vendors and then issue a
DESCRIBE
statement, MySQL reports the table structure using the equivalent
MySQL types. For example:
tbl_name
mysql>CREATE TABLE t (a BOOL, b FLOAT8, c LONG VARCHAR, d NUMERIC);Query OK, 0 rows affected (0.00 sec) mysql>DESCRIBE t;+-------+---------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------------+------+-----+---------+-------+ | a | tinyint(1) | YES | | NULL | | | b | double | YES | | NULL | | | c | mediumtext | YES | | NULL | | | d | decimal(10,0) | YES | | NULL | | +-------+---------------+------+-----+---------+-------+ 4 rows in set (0.01 sec)