expression1  IS [ NOT ] DISTINCT FROM expression2 

IS DISTINCT FROM 和 IS NOT DISTINCT FROM 搜索条件可优化搜索并可计算为 TRUE 或 FALSE。

如果 expression1 等于 expression2，或者两个表达式都为 NULL，则 IS NOT DISTINCT FROM 搜索条件计算为 TRUE。这相当于两个搜索条件的组合，如下所示：

expression1 = expression2 OR ( expression1 IS NULL AND expression2 IS NULL )

IS DISTINCT FROM 语法的含义相反。即如果 expression1 不等于 expression2，或者至少一个表达式不为 NULL，则 IS DISTINCT FROM 计算为 TRUE。这等效于以下语句：

NOT( expression1 = expression2 OR ( expression1 IS NULL AND expression2 IS NULL ))

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