A summary of the string data types follows. For additional information about properties of the string types, see Section 10.4, “String Types”. Storage requirements are given in Section 10.5, “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 12.1.14.1, “Silent Column Specification Changes”.
MySQL interprets length specifications in character column
definitions in character units. This applies to
CHAR
,
VARCHAR
, and the
TEXT
types.
Column definitions for many string data types can include
attributes that specify the character set or collation of the
column. These attributes apply to the
CHAR
,
VARCHAR
, the
TEXT
types,
ENUM
, and
SET
data types:
The CHARACTER SET
attribute specifies the
character set, and the COLLATE
attribute
specifies a collation for the character 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.
The rules for assigning the character set and collation when
either or both of the CHARACTER SET
and
COLLATE
attributes are missing are
described in Section 9.1.3.4, “Column Character Set and Collation”.
CHARSET
is a synonym for
CHARACTER SET
.
Specifying the CHARACTER SET binary
attribute for a character data type causes the column to be
created as the corresponding binary data type:
CHAR
becomes
BINARY
,
VARCHAR
becomes
VARBINARY
, and
TEXT
becomes
BLOB
. For the
ENUM
and
SET
data types, this does not
occur; they are created as declared. Suppose that you
specify a table using this definition:
CREATE TABLE t ( c1 VARCHAR(10) CHARACTER SET binary, c2 TEXT CHARACTER SET binary, c3 ENUM('a','b','c') CHARACTER SET binary );
The resulting table has this definition:
CREATE TABLE t ( c1 VARBINARY(10), c2 BLOB, c3 ENUM('a','b','c') CHARACTER SET binary );
The ASCII
attribute is shorthand for
CHARACTER SET latin1
.
The UNICODE
attribute is shorthand for
CHARACTER SET ucs2
.
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.
Character column sorting and comparison are based on the
character set assigned to the column. For the
CHAR
,
VARCHAR
,
TEXT
,
ENUM
, and
SET
data types, you can declare a
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.
Section 9.1, “Character Set Support”, provides additional information about use of character sets in MySQL.
[NATIONAL] CHAR[(
M
)]
[CHARACTER SET charset_name
]
[COLLATE
collation_name
]
A fixed-length string that is always right-padded with
spaces to the specified length when stored.
M
represents the column length in
characters. The range of M
is 0
to 255. If M
is omitted, the
length is 1.
Trailing spaces are removed when
CHAR
values are retrieved
unless the
PAD_CHAR_TO_FULL_LENGTH
SQL mode is enabled.
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 uses
utf8
as this predefined character set.
Section 9.1.3.6, “National Character Set”.
The CHAR BYTE
data type is an
alias for the BINARY
data
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
column that is defined as CHAR(0) NULL
occupies only one bit and can take only the values
NULL
and ''
(the empty
string).
[NATIONAL] VARCHAR(
M
)
[CHARACTER SET charset_name
]
[COLLATE
collation_name
]
A variable-length string. M
represents the maximum column length in characters. The
range of M
is 0 to 65,535. The
effective maximum length of a
VARCHAR
is subject to the
maximum row size (65,535 bytes, which is shared among all
columns) and the character set used. For example,
utf8
characters can require up to three
bytes per character, so a
VARCHAR
column that uses the
utf8
character set can be declared to be
a maximum of 21,844 characters.
MySQL stores VARCHAR
values
as a one-byte or two-byte length prefix plus data. The
length prefix indicates the number of bytes in the value. A
VARCHAR
column uses one
length byte if values require no more than 255 bytes, two
length bytes if values may require more than 255 bytes.
MySQL 5.5 follows the standard SQL
specification, and does not remove
trailing spaces from
VARCHAR
values.
VARCHAR
is shorthand for
CHARACTER VARYING
.
NATIONAL VARCHAR
is the
standard SQL way to define that a
VARCHAR
column should use
some predefined character set. MySQL 4.1 and up uses
utf8
as this predefined character set.
Section 9.1.3.6, “National Character Set”.
NVARCHAR
is shorthand for
NATIONAL VARCHAR
.
The BINARY
type is similar to
the CHAR
type, but stores
binary byte strings rather than nonbinary character strings.
M
represents the column length in
bytes.
The VARBINARY
type is similar
to the VARCHAR
type, but
stores binary byte strings rather than nonbinary character
strings. M
represents the maximum
column length in bytes.
A BLOB
column with a maximum
length of 255 (28 – 1)
bytes. Each TINYBLOB
value is
stored using a one-byte length prefix that indicates the
number of bytes in the value.
TINYTEXT [CHARACTER SET
charset_name
] [COLLATE
collation_name
]
A TEXT
column with a maximum
length of 255 (28 – 1)
characters. The effective maximum length is less if the
value contains multi-byte characters. Each
TINYTEXT
value is stored
using a one-byte length prefix that indicates the number of
bytes in the value.
A BLOB
column with a maximum
length of 65,535 (216 – 1)
bytes. Each BLOB
value is
stored using a two-byte length prefix that indicates the
number of bytes in the value.
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.
TEXT[(
M
)] [CHARACTER SET
charset_name
] [COLLATE
collation_name
]
A TEXT
column with a maximum
length of 65,535 (216 – 1)
characters. The effective maximum length is less if the
value contains multi-byte characters. Each
TEXT
value is stored using a
two-byte length prefix that indicates the number of bytes in
the value.
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. Each MEDIUMBLOB
value is stored using a three-byte length prefix that
indicates the number of bytes in the value.
MEDIUMTEXT [CHARACTER SET
charset_name
] [COLLATE
collation_name
]
A TEXT
column with a maximum
length of 16,777,215 (224 –
1) characters. The effective maximum length is less if the
value contains multi-byte characters. Each
MEDIUMTEXT
value is stored
using a three-byte length prefix that indicates the number
of bytes in the value.
A BLOB
column with a maximum
length of 4,294,967,295 or 4GB
(232 – 1) bytes. The
effective maximum length of
LONGBLOB
columns depends on
the configured maximum packet size in the client/server
protocol and available memory. Each
LONGBLOB
value is stored
using a four-byte length prefix that indicates the number of
bytes in the value.
LONGTEXT [CHARACTER SET
charset_name
] [COLLATE
collation_name
]
A TEXT
column with a maximum
length of 4,294,967,295 or 4GB
(232 – 1) characters. The
effective maximum length is less if the value contains
multi-byte characters. The effective maximum length of
LONGTEXT
columns also depends on the configured maximum packet size
in the client/server protocol and available memory. Each
LONGTEXT
value is stored using a four-byte length prefix that
indicates the number of bytes in the value.
ENUM('
value1
','value2
',...)
[CHARACTER SET charset_name
]
[COLLATE
collation_name
]
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.
SET('
value1
','value2
',...)
[CHARACTER SET charset_name
]
[COLLATE
collation_name
]
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.
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