댓글 0건 조회 5,799회 작성일 11-10-28 11:40
Every column of every table has an associated collating function. If no collating function is explicitly defined, then the collating function defaults to BINARY. The COLLATE clause of the column definition is used to define alternative collating functions for a column.
The rules for determining which collating function to use for a binary comparison operator (=, <, >, <=, >=, !=, IS, and IS NOT) are as follows and in the order shown:
If either operand has an explicit collating function assignment using the postfix COLLATE operator, then the explicit collating function is used for comparison, with precedence to the collating function of the left operand.
If either operand is a column, then the collating function of that column is used with precedence to the left operand. For the purposes of the previous sentence, a column name preceded by one or more unary "+" operators is still considered a column name.
Otherwise, the BINARY collating function is used for comparison.
An operand of a comparison is considered to have an explicit collating function assignment (rule 1 above) if any subexpression of the operand uses the postfix COLLATE operator. Thus, if a COLLATE operator is used anywhere in a comparision expression, the collating function defined by that operator is used for string comparison regardless of what table columns might be a part of that expression. If two or more COLLATE operator subexpressions appear anywhere in a comparison, the left most explicit collating function is used regardless of how deeply the COLLATE operators are nested in the expression and regardless of how the expression is parenthesized.
The expression "x BETWEEN y and z" is logically equivalent to two comparisons "x >= y AND x <= z" and works with respect to collating functions as if it were two separate comparisons. The expression "x IN (SELECT y ...)" is handled in the same way as the expression "x = y" for the purposes of determining the collating sequence. The collating sequence used for expressions of the form "x IN (y, z, ...)" is the collating sequence of x.
Terms of the ORDER BY clause that is part of a SELECT statement may be assigned a collating sequence using the COLLATE operator, in which case the specified collating function is used for sorting. Otherwise, if the expression sorted by an ORDER BY clause is a column, then the collating sequence of the column is used to determine sort order. If the expression is not a column and has no COLLATE clause, then the BINARY collating sequence is used.
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