Table 12.14 Cast Functions and Operators
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:
CONVERT(expr USING transcoding_name)
In MySQL, transcoding names are the same as the corresponding character set names.
SELECT CONVERT(_latin1'Müller' USING utf8); INSERT INTO utf8_table (utf8_column) SELECT CONVERT(latin1_column USING utf8) FROM latin1_table;
CONVERT(string, CHAR[(N)] CHARACTER SET charset_name) CAST(string AS CHAR[(N)] CHARACTER SET charset_name)
SELECT CONVERT('test', CHAR CHARACTER SET utf8); SELECT CAST('test' AS CHAR CHARACTER SET utf8);
If you specify
as just shown,
the resulting character set and collation are
charset_name and the default collation
charset_name. If you omit
, the resulting
character set and collation are defined by the
variables that determine the default connection character set and
collation (see Section 10.4, “Connection Character Sets and Collations”).
SELECT CAST('test' AS CHAR CHARACTER SET utf8) COLLATE utf8_bin;
But this is illegal:
SELECT CAST('test' AS CHAR CHARACTER SET utf8 COLLATE utf8_bin);
Normally, you cannot compare a
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
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:
SELECT 'A' LIKE CONVERT(blob_col USING latin1) 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
SELECT 'A' LIKE CONVERT(blob_col USING latin1) COLLATE latin1_german1_ci FROM tbl_name;
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:
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 | +---------------------------------+
For string literals, another way to specify the character set is
to use a character set introducer (
_latin2 in the preceding example are instances
of introducers). Unlike conversion functions such as
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.
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:
mysql> SET @str = BINARY 'New York'; mysql> SELECT LOWER(@str), LOWER(CONVERT(@str USING utf8mb4)); +-------------+------------------------------------+ | LOWER(@str) | LOWER(CONVERT(@str USING utf8mb4)) | +-------------+------------------------------------+ | New York | new york | +-------------+------------------------------------+
The cast functions are useful for creating a column with a
specific type in a
CREATE TABLE ...
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=utf8mb4
The cast functions are useful for sorting
ENUM columns in lexical order.
Normally, sorting of
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);
For temporal values, there is little need to use
CAST() to extract data in different
formats. Instead, use a function such as
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:
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 12.2, “Type Conversion in Expression Evaluation”.
MySQL supports arithmetic with both signed and unsigned 64-bit
values. For numeric operators (such as
-) 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,
cast operator to cast a value to a signed or unsigned 64-bit
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,
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 5.1.8, “Server SQL Modes”). Examples:
The following list describes the available cast functions and operators:
BINARYoperator converts the expression to a binary string. A common use for
BINARYis to force a character string comparison to be done byte by byte rather than character by character, in effect becoming case-sensitive. The
BINARYoperator also causes trailing spaces in comparisons to be significant.
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,
BINARYaffects 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:
BINARY expr CAST(expr AS BINARY) 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
_binarycharacter 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”.
BINARYoperator in expressions differs in effect from the
BINARYattribute in character column definitions. A character column defined with the
BINARYattribute is assigned table default character set and the binary (
_bin) collation of that character set. Every nonbinary character set has a
_bincollation. For example, the binary collation for the
utf8character set is
utf8_bin, so 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 binaryin the definition of a
TEXTcolumn 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
CAST()is standard SQL syntax.
CONVERT()function takes an expression of any type and produces a result value of the specified type.
USINGconverts 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
SELECT CONVERT('abc' USING utf8);
Produces a string with the
BINARYdata type. See Section 11.4.2, “The BINARY and VARBINARY Types” for a description of how this affects comparisons. If the optional length
BINARY(causes the cast to use no more than
Nbytes of the argument. Values shorter than
Nbytes are padded with
0x00bytes to a length of
Produces a string with the
CHARdata type. If the optional length
CHAR(causes the cast to use no more than
Ncharacters of the argument. No padding occurs for values shorter than
CHARproduces a string with the default character set. To specify the character set explicitly, these
charset_infovalues are permitted:
CHARACTER SET: 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.
DECIMALvalue. If the optional
Dvalues are given, they specify the maximum number of digits (the precision) and the number of digits following the decimal point (the scale).
CHAR, but produces a string with the national character set. See Section 10.3.7, “The National Character Set”.
NCHARdoes not permit trailing character set information to be specified.
Produces a signed integer value.
Produces an unsigned integer value.