12.10 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:

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

Examples:

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

Examples:

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:

But this is illegal:

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:

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:

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:

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

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:

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:

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:

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:

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

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:

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.

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.)

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: