2015. feb 11.

String Type Overview

írta: Sasy.
String Type Overview

String

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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 11.7, “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 13.1.14.3, “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 COLLATEattributes are missing are described in Section 10.1.3.4, “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 10.1, “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.

    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 4.1 and up uses utf8 as this predefined character set. Section 10.1.3.6, “National Character Set”.

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

    MySQL 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) NULLoccupies only one bit and can take only the values NULL and '' (the empty string).

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2015