MySQL 5.6 Reference Manual Including MySQL NDB Cluster 7.3-7.4 Reference Guide
Cast functions and operators enable conversion of values from one data type to another.
CONVERT()
with a
USING
clause converts data between character
sets:
CONVERT(expr
USINGtranscoding_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(
syntax (without
expr
,
type
)USING
), or
CAST(
, which is equivalent:
expr
AS
type
)
CONVERT(string
, CHAR[(N
)] CHARACTER SETcharset_name
) CAST(string
AS CHAR[(N
)] CHARACTER SETcharset_name
)
Examples:
SELECT CONVERT('test', CHAR CHARACTER SET utf8); SELECT CAST('test' AS CHAR CHARACTER SET utf8);
If you specify CHARACTER SET
as just shown,
the character set and collation of the result are
charset_name
charset_name
and the default collation
of charset_name
. If you omit
CHARACTER SET
, the character
set and collation of the result are defined by the
charset_name
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);
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) FROMtbl_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 FROMtbl_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 |
+---------------------------------+
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 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 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:
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:
SELECTenum_col
FROMtbl_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.
The following list describes the available cast functions and operators:
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:
CAST(expr
AS BINARY) CONVERT(expr
USING BINARY) BINARYexpr
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
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()
.
CONVERT(
,
expr
USING transcoding_name
)CONVERT(
expr
,type
)
The CONVERT()
function takes an
expression of any type and produces a result value of the
specified type.
CONVERT(... USING ...)
is
standard SQL syntax. The non-USING
form of
CONVERT()
is ODBC syntax.
CONVERT(
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
expr
USING transcoding_name
)'abc'
in the default character set to the
corresponding string in the utf8
character
set:
SELECT CONVERT('abc' USING utf8);
CONVERT(
syntax (without
expr
,
type
)USING
) takes an expression and a
type
value specifying the result
type. This operation may also be expressed as
CAST(
, which is
equivalent. These expr
AS
type
)type
values are
permitted:
BINARY[(
N
)]
Produces a string with the
BINARY
data type. For a
description of how this affects comparisons, see
Section 11.3.3, “The BINARY and VARBINARY Types”. If the optional length
N
is given,
BINARY(
causes the cast to use no more than
N
)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(
causes the cast to use no more than
N
)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
:
Produces a string with the given character set.
charset_name
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
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).
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 integer value.
TIME
Produces a TIME
value.
UNSIGNED [INTEGER]
Produces an unsigned integer value.