MySQL 8.0 Reference Manual Including MySQL NDB Cluster 8.0

12.11 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.

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 utf8mb4);
SELECT CONVERT(_latin1'Müller' USING utf8mb4);
INSERT INTO utf8mb4_table (utf8mb4_column)
    SELECT CONVERT(latin1_column USING utf8mb4) 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 utf8mb4);
SELECT CAST('test' AS CHAR CHARACTER SET utf8mb4);

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 utf8mb4) COLLATE utf8mb4_bin;
SELECT CONVERT('test', CHAR CHARACTER SET utf8mb4) COLLATE utf8mb4_bin;
SELECT CAST('test' AS CHAR CHARACTER SET utf8mb4) COLLATE utf8mb4_bin;

But these are illegal: 

SELECT CONVERT('test' USING utf8mb4 COLLATE utf8mb4_bin);
SELECT CONVERT('test', CHAR CHARACTER SET utf8mb4 COLLATE utf8mb4_bin);
SELECT CAST('test' AS CHAR CHARACTER SET utf8mb4 COLLATE utf8mb4_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 utf8mb4 collation (utf8mb4_0900_ai_ci) is not case-sensitive: 

SELECT 'A' LIKE CONVERT(blob_col USING utf8mb4)
  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 utf8mb4) COLLATE utf8mb4_unicode_ci
  FROM tbl_name;

To use a different character set, substitute its name for utf8mb4 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 utf8mb4));
+-------------+------------------------------------+
| LOWER(@str) | LOWER(CONVERT(@str USING utf8mb4)) |
+-------------+------------------------------------+
| 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=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: 

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 5.1.11, “Server SQL Modes”). Examples:

The following list describes the available cast functions and operators:

MySQL Documentation Library