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10.5.4. Some Special Cases Where the Collation Determination Is Tricky

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 x:

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

However, when multiple operands are involved, there can be ambiguity. For example:

SELECT x FROM T WHERE x = 'Y';

Should this query use the collation of the column x, or of the string literal 'Y'?

Standard SQL resolves such questions using what used to be called “coercibility” rules. Basically, this means: Both x and 'Y' have collations, so which collation takes precedence? This can be difficult to resolve, but the following rules cover most situations:

  • An explicit COLLATE clause has a coercibility of 0. (Not coercible at all.)

  • The concatenation of two strings with different collations has a coercibility of 1.

  • The collation of a column or a stored routine parameter or local variable has a coercibility of 2.

  • A “system constant” (the string returned by functions such as USER() or VERSION()) has a coercibility of 3.

  • A literal's collation has a coercibility of 4.

  • NULL or an expression that is derived from NULL has a coercibility of 5.

The preceding coercibility values are current for MySQL 5.1.

Those rules resolve ambiguities in the following manner:

  • Use the collation with the lowest coercibility value.

  • If both sides have the same coercibility, then it is an error if the collations aren't the same.

Examples:

column1 = 'A' Use collation of column1
column1 = 'A' COLLATE x Use collation of 'A'
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);
        -> 0
mysql> SELECT COERCIBILITY(VERSION());
        -> 3
mysql> SELECT COERCIBILITY('A');
        -> 4

See Section 12.10.3, “Information Functions”.


 
 
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