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12.10 Cast Functions and Operators

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Table 12.14 Cast Functions and Operators

Name Description
BINARY Cast a string to a binary string
CAST() Cast a value as a certain type
CONVERT() Cast a value as a certain type

Cast functions and operators enable conversion of values from one data type to another.

CONVERT() with a USING clause provides a way to convert data between different character sets:

  1. CONVERT(expr USING transcoding_name)

In MySQL, transcoding names are the same as the corresponding character set names.

Examples:

  1. SELECT CONVERT(_latin1'Müller' USING utf8);
  2. INSERT INTO utf8_table (utf8_column)
  3.     SELECT CONVERT(latin1_column USING utf8) FROM latin1_table;

You can also use CONVERT() without USING or CAST() to convert strings between different character sets:

  1. CONVERT(string, CHAR[(N)] CHARACTER SET charset_name)
  2. CAST(string AS CHAR[(N)] CHARACTER SET charset_name)

Examples:

  1. SELECT CONVERT('test', CHAR CHARACTER SET utf8);
  2. SELECT CAST('test' AS CHAR CHARACTER SET utf8);

If you specify CHARACTER SET charset_name as just shown, the resulting character set and collation are charset_name and the default collation of charset_name. If you omit CHARACTER SET charset_name, the resulting character set and collation are defined by the character_set_connection and collation_connection system variables that determine the default connection character set and collation (see Section 10.4, “Connection Character Sets and Collations”).

A COLLATE clause is not permitted within a CONVERT() or CAST() call, but you can apply it to the function result. For example, this is legal:

  1. SELECT CAST('test' AS CHAR CHARACTER SET utf8) COLLATE utf8_bin;

But this is illegal:

  1. SELECT CAST('test' AS CHAR CHARACTER SET utf8 COLLATE utf8_bin);

Normally, you cannot compare a BLOB value or other binary string in case-insensitive fashion because binary strings use the binary character set, which has no collation with the concept of lettercase. To perform a case-insensitive comparison, use the CONVERT() or CAST() function to convert the value to a nonbinary string. Comparisons of the resulting string use its collation. For example, if the conversion result character set has a case-insensitive collation, a LIKE operation is not case-sensitive:

  1. SELECT 'A' LIKE CONVERT(blob_col USING latin1)
  2.   FROM tbl_name;

To use a different character set, substitute its name for latin1 in the preceding statement. To specify a particular collation for the converted string, use a COLLATE clause following the CONVERT() call:

  1. SELECT 'A' LIKE CONVERT(blob_col USING latin1) COLLATE latin1_german1_ci
  2.   FROM tbl_name;

CONVERT() and CAST() can be used more generally for comparing strings that are represented in different character sets. For example, a comparison of these strings results in an error because they have different character sets:

  1. mysql> SET @s1 = _latin1 'abc', @s2 = _latin2 'abc';
  2. mysql> SELECT @s1 = @s2;
  3. ERROR 1267 (HY000): Illegal mix of collations (latin1_swedish_ci,IMPLICIT)
  4. and (latin2_general_ci,IMPLICIT) for operation '='

Converting one of the strings to a character set compatible with the other enables the comparison to occur without error:

  1. mysql> SELECT @s1 = CONVERT(@s2 USING latin1);
  2. +---------------------------------+
  3. | @s1 = CONVERT(@s2 USING latin1) |
  4. +---------------------------------+
  5. |                               1 |
  6. +---------------------------------+

For string literals, another way to specify the character set is to use a character set introducer (_latin1 and _latin2 in the preceding example are instances of introducers). Unlike conversion functions such as CAST(), or CONVERT(), which convert a string from one character set to another, an introducer designates a string literal as having a particular character set, with no conversion involved. For more information, see Section 10.3.8, “Character Set Introducers”.

Character set conversion is also useful preceding lettercase conversion of binary strings. LOWER() and UPPER() are ineffective when applied directly to binary strings because the concept of lettercase does not apply. To perform lettercase conversion of a binary string, first convert it to a nonbinary string:

  1. mysql> SET @str = BINARY 'New York';
  2. mysql> SELECT LOWER(@str), LOWER(CONVERT(@str USING utf8mb4));
  3. +-------------+------------------------------------+
  4. | LOWER(@str) | LOWER(CONVERT(@str USING utf8mb4)) |
  5. +-------------+------------------------------------+
  6. | New York    | new york                           |
  7. +-------------+------------------------------------+

If you convert an indexed column using BINARY, CAST(), or CONVERT(), MySQL may not be able to use the index efficiently.

The cast functions are useful for creating a column with a specific type in a CREATE TABLE ... SELECT statement:

  1. mysql> CREATE TABLE new_table SELECT CAST('2000-01-01' AS DATE) AS c1;
  2. mysql> SHOW CREATE TABLE new_table\G
  3. *************************** 1. row ***************************
  4.        Table: new_table
  5. Create Table: CREATE TABLE `new_table` (
  6.   `c1` date DEFAULT NULL
  7. ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4

The cast functions are useful for sorting ENUM columns in lexical order. Normally, sorting of ENUM columns occurs using the internal numeric values. Casting the values to CHAR results in a lexical sort:

  1. SELECT enum_col FROM tbl_name ORDER BY CAST(enum_col AS CHAR);

CAST() also changes the result if you use it as part of a more complex expression such as CONCAT('Date: ',CAST(NOW() AS DATE)).

For temporal values, there is little need to use CAST() to extract data in different formats. Instead, use a function such as EXTRACT(), DATE_FORMAT(), or TIME_FORMAT(). See Section 12.7, “Date and Time Functions”.

To cast a string to a number, you normally need do nothing other than use the string value in numeric context:

  1. mysql> SELECT 1+'1';
  2.        -> 2

That is also true for hexadecimal and bit literals, which are binary strings by default:

  1. mysql> SELECT X'41', X'41'+0;
  2.         -> 'A', 65
  3. mysql> SELECT b'1100001', b'1100001'+0;
  4.         -> 'a', 97

A string used in an arithmetic operation is converted to a floating-point number during expression evaluation.

A number used in string context is converted to a string:

  1. mysql> SELECT CONCAT('hello you ',2);
  2.         -> 'hello you 2'

For information about implicit conversion of numbers to strings, see Section 12.2, “Type Conversion in Expression Evaluation”.

MySQL supports arithmetic with both signed and unsigned 64-bit values. For numeric operators (such as + or -) where one of the operands is an unsigned integer, the result is unsigned by default (see Section 12.6.1, “Arithmetic Operators”). To override this, use the SIGNED or UNSIGNED cast operator to cast a value to a signed or unsigned 64-bit integer, respectively.

  1. mysql> SELECT 1 - 2;
  2.         -> -1
  3. mysql> SELECT CAST(1 - 2 AS UNSIGNED);
  4.         -> 18446744073709551615
  5. mysql> SELECT CAST(CAST(1 - 2 AS UNSIGNED) AS SIGNED);
  6.         -> -1

If either operand is a floating-point value, the result is a floating-point value and is not affected by the preceding rule. (In this context, DECIMAL column values are regarded as floating-point values.)

  1. mysql> SELECT CAST(1 AS UNSIGNED) - 2.0;
  2.         -> -1.0

The SQL mode affects the result of conversion operations (see Section 5.1.11, “Server SQL Modes”). Examples:

  • For conversion of a “zero” date string to a date, CONVERT() and CAST() return NULL and produce a warning when the NO_ZERO_DATE SQL mode is enabled.

  • For integer subtraction, if the NO_UNSIGNED_SUBTRACTION SQL mode is enabled, the subtraction result is signed even if any operand is unsigned.

The following list describes the available cast functions and operators:

  • BINARY expr

    The BINARY operator converts the expression to a binary string. A common use for BINARY is to force a character string comparison to be done byte by byte rather than character by character, in effect becoming case-sensitive. The BINARY operator also causes trailing spaces in comparisons to be significant.

    1. mysql> SELECT 'a' = 'A';
    2.         -> 1
    3. mysql> SELECT BINARY 'a' = 'A';
    4.         -> 0
    5. mysql> SELECT 'a' = 'a ';
    6.         -> 1
    7. mysql> SELECT BINARY 'a' = 'a ';
    8.         -> 0

    In a comparison, BINARY affects the entire operation; it can be given before either operand with the same result.

    For purposes of converting a string expression to a binary string, these constructs are equivalent:

    1. BINARY expr
    2. CAST(expr AS BINARY)
    3. CONVERT(expr USING BINARY)

    If a value is a string literal, it can be designated as a binary string without performing any conversion by using the _binary character set introducer:

    1. mysql> SELECT 'a' = 'A';
    2.         -> 1
    3. mysql> SELECT _binary 'a' = 'A';
    4.         -> 0

    For information about introducers, see Section 10.3.8, “Character Set Introducers”.

    The BINARY operator in expressions differs in effect from the BINARY attribute in character column definitions. A character column defined with the BINARY attribute is assigned table default character set and the binary (_bin) collation of that character set. Every nonbinary character set has a _bin collation. For example, the binary collation for the utf8 character set is utf8_bin, so if the table default character set is utf8, these two column definitions are equivalent:

    1. CHAR(10) BINARY
    2. CHAR(10) CHARACTER SET utf8 COLLATE utf8_bin

    The use of CHARACTER SET binary in the definition of a CHAR, VARCHAR, or TEXT column causes the column to be treated as the corresponding binary string data type. For example, the following pairs of definitions are equivalent:

    1. CHAR(10) CHARACTER SET binary
    2. BINARY(10)
    3.  
    4. VARCHAR(10) CHARACTER SET binary
    5. VARBINARY(10)
    6.  
    7. TEXT CHARACTER SET binary
    8. BLOB

  • CAST(expr AS type [ARRAY])

    The CAST() function takes an expression of any type and produces a result value of the specified type, similar to CONVERT(). For more information, see the description of CONVERT().

    In MySQL 8.0.17 and later, InnoDB allows the use of an additional ARRAY keyword for creating a multi-valued index on a JSON array as part of CREATE INDEX, CREATE TABLE, and ALTER TABLE statements. ARRAY is not supported except when used to create a multi-valued index in one of these statements, in which case it is required. The column being indexed must be a column of type JSON. (CONVERT() does not support multi-valued index creation or the ARRAY keyword.) The type following the AS keyword may be any of the types supported by CAST(), with the exception of BINARY, which is not supported for this purpose. Multi-Valued Indexes, provides syntax information and examples, as well as other relevant information.

    CAST() is standard SQL syntax.

  • CONVERT(expr,type), CONVERT(expr USING transcoding_name)

    The CONVERT() function takes an expression of any type and produces a result value of the specified type.

    Discussion of CONVERT(expr, type) syntax here also applies to CAST(expr AS type), which is equivalent.

    CONVERT(... USING ...) is standard SQL syntax. The non-USING form of CONVERT() is ODBC syntax.

    CONVERT() with USING converts data between different character sets. In MySQL, transcoding names are the same as the corresponding character set names. For example, this statement converts the string 'abc' in the default character set to the corresponding string in the utf8 character set:

    1. SELECT CONVERT('abc' USING utf8);

    CONVERT() without USING and CAST() take an expression and a type value specifying the result type. These type values are permitted:

    • BINARY[(N)]

      Produces a string with the BINARY data type. See Section 11.4.2, “The BINARY and VARBINARY Types” for a description of how this affects comparisons. If the optional length N is given, BINARY(N) causes the cast to use no more than N bytes of the argument. Values shorter than N bytes are padded with 0x00 bytes to a length of N.

    • CHAR[(N)] [charset_info]

      Produces a string with the CHAR data type. If the optional length N is given, CHAR(N) causes the cast to use no more than N characters of the argument. No padding occurs for values shorter than N characters.

      With no charset_info clause, CHAR produces a string with the default character set. To specify the character set explicitly, these charset_info values are permitted:

      • CHARACTER SET charset_name: Produces a string with the given character set.

      • ASCII: Shorthand for CHARACTER SET latin1.

      • UNICODE: Shorthand for CHARACTER SET ucs2.

      In all cases, the string has the default collation for the character set.

    • DATE

      Produces a DATE value.

    • DATETIME

      Produces a DATETIME value.

    • DECIMAL[(M[,D])]

      Produces a DECIMAL value. If the optional M and D values are given, they specify the maximum number of digits (the precision) and the number of digits following the decimal point (the scale).

    • DOUBLE

      Produces a DOUBLE result. Added in MySQL 8.0.17.

    • FLOAT[(P)]

      If the precision P is not specified, produces a result of type FLOAT. If P is provided and 0 <= < P <= 24, the result is of type FLOAT. If 25 <= P <= 53, the result is of type REAL. If P < 0 or P > 53, an error is returned. Added in MySQL 8.0.17.

    • JSON

      Produces a JSON value. For details on the rules for conversion of values between JSON and other types, see Comparison and Ordering of JSON Values.

    • NCHAR[(N)]

      Like CHAR, but produces a string with the national character set. See Section 10.3.7, “The National Character Set”.

      Unlike CHAR, NCHAR does not permit trailing character set information to be specified.

    • REAL

      Produces a result of type REAL. This is actually FLOAT if REAL_AS_FLOAT SQL mode is enabled; otherwise the result is of type DOUBLE.

    • SIGNED [INTEGER]

      Produces a signed integer value.

    • TIME

      Produces a TIME value.

    • UNSIGNED [INTEGER]

      Produces an unsigned integer value.


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