12.10 Cast Functions and Operators

Table 12.15 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.

• Cast Function and Operator Descriptions

• Character Set Conversions

• Character Set Conversions for String Comparisons

• Other Uses for Cast Operations

Cast Function and Operator Descriptions

• BINARY expr

  The BINARY operator converts the expression to a binary string (a string that has the binary character set and binary collation). A common use for BINARY is to force a character string comparison to be done byte by byte using numeric byte values rather than character by character. The BINARY operator also causes trailing spaces in comparisons to be significant. For information about the differences between the binary collation of the binary character set and the _bin collations of nonbinary character sets, see Section 10.8.5, “The binary Collation Compared to _bin Collations”.

  mysql> SELECT 'a' = 'A';
          -> 1
  mysql> SELECT BINARY 'a' = 'A';
          -> 0
  mysql> SELECT 'a' = 'a ';
          -> 1
  mysql> SELECT BINARY 'a' = 'a ';
          -> 0

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

  To convert a string expression to a binary string, these constructs are equivalent:

  CONVERT(expr USING BINARY)
  CAST(expr AS BINARY)
  BINARY expr

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

  mysql> SELECT 'a' = 'A';
          -> 1
  mysql> SELECT _binary 'a' = 'A';
          -> 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. For a character column defined with the BINARY attribute, MySQL assigns the table default character set and the binary (_bin) collation of that character set. Every nonbinary character set has a _bin collation. For example, if the table default character set is utf8, these two column definitions are equivalent:

  CHAR(10) BINARY
  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:

  CHAR(10) CHARACTER SET binary
  BINARY(10)
  
  VARCHAR(10) CHARACTER SET binary
  VARBINARY(10)
  
  TEXT CHARACTER SET binary
  BLOB

  If BINARY is invoked from within the mysql client, binary strings display using hexadecimal notation, depending on the value of the --binary-as-hex. For more information about that option, see Section 4.5.1, “mysql — The MySQL Command-Line Client”.

• CAST(expr AS type)

  CAST(expr AS type takes an expression of any type and produces a result value of the specified type. This operation may also be expressed as CONVERT(expr, type), which is equivalent.

  These type values are permitted:

  • BINARY[(N)]

    Produces a string with the VARBINARY data type, except that when the expression expr is empty (zero length), the result type is BINARY(0). 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. If the optional length N is not given, MySQL calculates the maximum length from the expression. If the supplied or calculated length is greater than an internal threshold, the result type is BLOB. If the length is still too long, the result type is LONGBLOB.

    For a description of how casting to BINARY affects comparisons, see Section 11.3.3, “The BINARY and VARBINARY Types”.

  • CHAR[(N)] [charset_info]

    Produces a string with the VARCHAR data type, unless the expression expr is empty (zero length), in which case the result type is CHAR(0). 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. If the optional length N is not given, MySQL calculates the maximum length from the expression. If the supplied or calculated length is greater than an internal threshold, the result type is TEXT. If the length is still too long, the result type is LONGTEXT.

    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 character set default collation.

  • DATE

    Produces a DATE value.

  • DATETIME[(M)]

    Produces a DATETIME value. If the optional M value is given, it specifies the fractional seconds precision.

  • 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). If D is omitted, 0 is assumed. If M is omitted, 10 is assumed.

  • 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.

  • SIGNED [INTEGER]

    Produces a signed BIGINT value.

  • TIME[(M)]

    Produces a TIME value. If the optional M value is given, it specifies the fractional seconds precision.

  • UNSIGNED [INTEGER]

    Produces an unsigned BIGINT value.

• CONVERT(expr USING transcoding_name)

  CONVERT(expr,type)

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

  CONVERT(expr USING transcoding_name) 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:

  SELECT CONVERT('abc' USING utf8);

  CONVERT(expr, type) syntax (without USING) takes an expression and a type value specifying a result type, and produces a result value of the specified type. This operation may also be expressed as CAST(expr AS type), which is equivalent. For more information, see the description of CAST().

Character Set Conversions

CONVERT() with a USING clause converts data between character sets:

CONVERT(expr USING transcoding_name)

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

Examples:

SELECT CONVERT('test' USING utf8);
SELECT CONVERT(_latin1'Müller' USING utf8);
INSERT INTO utf8_table (utf8_column)
    SELECT CONVERT(latin1_column USING utf8) FROM latin1_table;

To convert strings between character sets, you can also use CONVERT(expr, type) syntax (without USING), or CAST(expr AS type), which is equivalent:

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

Examples:

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

If you specify CHARACTER SET charset_name as just shown, the character set and collation of the result are charset_name and the default collation of charset_name. If you omit CHARACTER SET charset_name, the character set and collation of the result 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, these are legal:

SELECT CONVERT('test' USING utf8) COLLATE utf8_bin;
SELECT CONVERT('test', CHAR CHARACTER SET utf8) COLLATE utf8_bin;
SELECT CAST('test' AS CHAR CHARACTER SET utf8) COLLATE utf8_bin;

But these are illegal:

SELECT CONVERT('test' USING utf8 COLLATE utf8_bin);
SELECT CONVERT('test', CHAR CHARACTER SET utf8 COLLATE utf8_bin);
SELECT CAST('test' AS CHAR CHARACTER SET utf8 COLLATE utf8_bin);

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 Conversions for String Comparisons

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, first 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 collation is not case-sensitive, a LIKE operation is not case-sensitive. That is true for the following operation because the default latin1 collation (latin1_swedish_ci) is not case-sensitive:

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

To specify a particular collation for the converted string, use a COLLATE clause following the CONVERT() call:

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

To use a different character set, substitute its name for latin1 in the preceding statements (and similarly to use a different collation).

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

mysql> SET @s1 = _latin1 'abc', @s2 = _latin2 'abc';
mysql> SELECT @s1 = @s2;
ERROR 1267 (HY000): Illegal mix of collations (latin1_swedish_ci,IMPLICIT)
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:

mysql> SELECT @s1 = CONVERT(@s2 USING latin1);
+---------------------------------+
| @s1 = CONVERT(@s2 USING latin1) |
+---------------------------------+
|                               1 |
+---------------------------------+

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 using a character set appropriate for the data stored in the string:

mysql> SET @str = BINARY 'New York';
mysql> SELECT LOWER(@str), LOWER(CONVERT(@str USING latin1));
+-------------+-----------------------------------+
| LOWER(@str) | LOWER(CONVERT(@str USING latin1)) |
+-------------+-----------------------------------+
| New York    | new york                          |
+-------------+-----------------------------------+

Be aware that if you apply BINARY, CAST(), or CONVERT() to an indexed column, MySQL may not be able to use the index efficiently.

Other Uses for Cast Operations

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

mysql> CREATE TABLE new_table SELECT CAST('2000-01-01' AS DATE) AS c1;
mysql> SHOW CREATE TABLE new_table\G
*************************** 1. row ***************************
       Table: new_table
Create Table: CREATE TABLE `new_table` (
  `c1` date DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1

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:

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, it normally suffices to use the string value in numeric context:

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

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

mysql> SELECT X'41', X'41'+0;
        -> 'A', 65
mysql> SELECT b'1100001', b'1100001'+0;
        -> '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:

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

For information about implicit conversion of numbers to strings, see Section 12.3, “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.

mysql> SELECT 1 - 2;
        -> -1
mysql> SELECT CAST(1 - 2 AS UNSIGNED);
        -> 18446744073709551615
mysql> SELECT CAST(CAST(1 - 2 AS UNSIGNED) AS SIGNED);
        -> -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.)

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

The SQL mode affects the result of conversion operations (see Section 5.1.10, “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.