MySQL 8.0 Reference Manual Including MySQL NDB Cluster 8.0

10.8.4 Collation Coercibility in 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 x:

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?

A mix of collations may also occur in contexts other than comparison. For example, a multiple-argument concatenation operation such as CONCAT(x,'Y') combines its arguments to produce a single string. What collation should the result have?

To resolve questions like these, MySQL checks whether the collation of one item can be coerced to the collation of the other. 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 standard 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.

The following table illustrates some applications of the preceding rules.

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

To determine the coercibility of a string expression, use the COERCIBILITY() function (see Section 12.15, “Information Functions”):

mysql> SELECT COERCIBILITY('A' COLLATE latin1_swedish_ci);
        -> 0
mysql> SELECT COERCIBILITY(VERSION());
        -> 3
mysql> SELECT COERCIBILITY('A');
        -> 4
mysql> SELECT COERCIBILITY(1000);
        -> 5

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”.