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11.1.3 String Type Overview

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A summary of the string data types follows. For additional information about properties and storage requirements of the string types, see Section 11.4, “String Types”, and Section 11.8, “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 13.1.20.8, “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 character string data types CHAR, VARCHAR, the TEXT types, ENUM, SET, and any synonyms) can specify the column character set and collation:

  • CHARACTER SET specifies the character set. If desired, a collation for the character set can be specified with the COLLATE attribute, along with any other attributes. For example:

    1. CREATE TABLE t
    2. (
    3.     c1 VARCHAR(20) CHARACTER SET utf8,
    4.     c2 TEXT CHARACTER SET latin1 COLLATE latin1_general_cs
    5. );

    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 CHARACTER SET and the COLLATE attribute are missing are described in Section 10.3.5, “Column Character Set and Collation”.

    CHARSET is a synonym for CHARACTER SET.

  • Specifying the CHARACTER SET binary attribute for a character string data type causes the column to be created as the corresponding binary string 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:

    1. CREATE TABLE t
    2. (
    3.   c1 VARCHAR(10) CHARACTER SET binary,
    4.   c2 TEXT CHARACTER SET binary,
    5.   c3 ENUM('a','b','c') CHARACTER SET binary
    6. );

    The resulting table has this definition:

    1. CREATE TABLE t
    2. (
    3.   c1 VARBINARY(10),
    4.   c2 BLOB,
    5.   c3 ENUM('a','b','c') CHARACTER SET binary
    6. );

  • The BINARY attribute is shorthand for specifying the table default character set and the binary (_bin) collation of that character set. In this case, comparison and sorting are based on numeric character code values.

  • The ASCII attribute is shorthand for CHARACTER SET latin1.

  • The UNICODE attribute is shorthand for CHARACTER SET ucs2.

Character column comparison and sorting are based on the collation assigned to the column. For the CHAR, VARCHAR, TEXT, ENUM, and SET data types, you can declare a column with a binary (_bin) collation or the BINARY attribute to cause comparison and sorting to use the underlying character code values rather than a lexical ordering.

For additional information about use of character sets in MySQL, see Chapter 10, Character Sets, Collations, Unicode.

  • [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.

    Note

    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 uses utf8 as this predefined character set. Section 10.3.7, “The National Character Set”.

    The CHAR BYTE data type is an alias for the BINARY data type. This is a compatibility feature.

    MySQL permits 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. See Section C.10.4, “Limits on Table Column Count and Row Size”.

    MySQL stores VARCHAR values as a 1-byte or 2-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.

    Note

    MySQL 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 uses utf8 as this predefined character set. Section 10.3.7, “The National Character Set”. NVARCHAR is shorthand for NATIONAL VARCHAR.

  • BINARY[(M)]

    The BINARY type is similar to the CHAR type, but stores binary byte strings rather than nonbinary character strings. An optional length M represents the column length in bytes. If omitted, M defaults to 1.

  • VARBINARY(M)

    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.

  • TINYBLOB

    A BLOB column with a maximum length of 255 (28 − 1) bytes. Each TINYBLOB value is stored using a 1-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 multibyte characters. Each TINYTEXT value is stored using a 1-byte length prefix that indicates the number of bytes in the value.

  • BLOB[(M)]

    A BLOB column with a maximum length of 65,535 (216 − 1) bytes. Each BLOB value is stored using a 2-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 multibyte characters. Each TEXT value is stored using a 2-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.

  • MEDIUMBLOB

    A BLOB column with a maximum length of 16,777,215 (224 − 1) bytes. Each MEDIUMBLOB value is stored using a 3-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 multibyte characters. Each MEDIUMTEXT value is stored using a 3-byte length prefix that indicates the number of bytes in the value.

  • LONGBLOB

    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 4-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 multibyte 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 4-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. ENUM values are represented internally as integers.

    An ENUM column can have a maximum of 65,535 distinct elements.

    The maximum supported length of an individual ENUM element is M <= 255 and (M x w) <= 1020, where M is the element literal length and w is the number of bytes required for the maximum-length character in the character set.

  • 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', ... SET values are represented internally as integers.

    A SET column can have a maximum of 64 distinct members.

    The maximum supported length of an individual SET element is M <= 255 and (M x w) <= 1020, where M is the element literal length and w is the number of bytes required for the maximum-length character in the character set.


Rechercher dans le manuel MySQL

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