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10.1.7.5. Collation of Expressions

In the great majority of statements, it is obvious what collation MySQL uses to resolve a comparison operation. For example, in the following cases, it should be clear that the collation is the collation of column charset_name:

SELECT x FROM T ORDER BY x;SELECT x FROM T WHERE x = x;SELECT DISTINCT x FROM T;

However, with multiple operands, there can be ambiguity. For example:

SELECT x FROM T WHERE x = 'Y';

Should the comparison use the collation of the column x, or of the string literal 'Y'? Both x and 'Y' have collations, so which collation takes precedence?

Standard SQL resolves such questions using what used to be called "coercibility" rules. MySQL assigns coercibility values as follows:

MySQL uses coercibility values with the following rules to resolve ambiguities:

Although automatic conversion is not in the SQL standard, the SQL standard document does say that every character set is (in terms of supported characters) a "subset" of Unicode. Because it is a well-known principle that "what applies to a superset can apply to a subset," we believe that a collation for Unicode can apply for comparisons with non-Unicode strings.

Examples:

Comparison Collation Used
column1 = 'A' Use collation of column1
column1 = 'A' COLLATE x Use collation of 'A' COLLATE x
column1 COLLATE x = 'A' COLLATE y Error

The COERCIBILITY() function can be used to determine the coercibility of a string expression:

mysql> SELECT COERCIBILITY('A' COLLATE
        latin1_swedish_ci);        -> 0mysql> SELECT
        COERCIBILITY(VERSION());        -> 3mysql> SELECT
        COERCIBILITY('A');        -> 4

See Section 12.14, "Information Functions".

For implicit conversion of a numeric or temporal value to a string, such as occurs for the argument 1 in the expression CONCAT(1, 'abc'), the result is a character (nonbinary) string that has a character set and collation determined by the character_set_connection and collation_connection system variables. See Section 12.2, "Type Conversion in Expression Evaluation".