MySQL :: MySQL 8.4 Reference Manual :: 12.8.4 Collation Coercibility in Expressions

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12.8.4 表达式中的排序规则强制性

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, this statement performs a comparison between the column x and the string literal 'Y':

SELECT x FROM T WHERE x = 'Y';

If x and 'Y' have the same collation, there is no ambiguity about the collation to use for the comparison. But if they have different collations, should the comparison use the collation of x, or of '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:

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.
The collation of a literal has a coercibility of 4.
The collation of a numeric or temporal value has a coercibility of 5.
NULL or an expression that is derived from NULL has a coercibility of 6.

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

Use the collation with the lowest coercibility value.
If both sides have the same coercibility, then:
- If both sides are Unicode, or both sides are not Unicode, it is an error.
- If one of the sides has a Unicode character set, and another side has a non-Unicode character set, the side with Unicode character set wins, and automatic character set conversion is applied to the non-Unicode side. For example, the following statement does not return an error:
```
SELECT CONCAT(utf8mb4_column, latin1_column) FROM t1;
```
  It returns a result that has a character set of utf8mb4 and the same collation as utf8mb4_column. Values of latin1_column are automatically converted to utf8mb4 before concatenating.
- For an operation with operands from the same character set but that mix a _bin collation and a _ci or _cs collation, the _bin collation is used. This is similar to how operations that mix nonbinary and binary strings evaluate the operands as binary strings, applied to collations rather than data types.

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. More generally, MySQL uses the concept of character set repertoire, which can sometimes be used to determine subset relationships among character sets and enable conversion of operands in operations that would otherwise produce an error. See Section 12.2.1, “Character Set Repertoire”.

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 14.15, “Information Functions”):

mysql> SELECT COERCIBILITY(_utf8mb4'A' COLLATE utf8mb4_bin);
        -> 0
mysql> SELECT COERCIBILITY(VERSION());
        -> 3
mysql> SELECT COERCIBILITY('A');
        -> 4
mysql> SELECT COERCIBILITY(1000);
        -> 5
mysql> SELECT COERCIBILITY(NULL);
        -> 6

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 14.3, “Type Conversion in Expression Evaluation”.

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