# 9.1. 邏輯運算子

常見可用的邏輯運算子：

| `AND` |
| ----- |
| `OR`  |
| `NOT` |

SQL 使用具有 true、false 和 null 的三值邏輯系統，其中 null 表示“未知”。請參閱以下真值表：

| *`a`* | *`b`* | *`a`* AND *`b`* | *`a`* OR *`b`* |
| ----- | ----- | --------------- | -------------- |
| TRUE  | TRUE  | TRUE            | TRUE           |
| TRUE  | FALSE | FALSE           | TRUE           |
| TRUE  | NULL  | NULL            | TRUE           |
| FALSE | FALSE | FALSE           | FALSE          |
| FALSE | NULL  | FALSE           | NULL           |
| NULL  | NULL  | NULL            | NULL           |

| *`a`* | NOT *`a`* |
| ----- | --------- |
| TRUE  | FALSE     |
| FALSE | TRUE      |
| NULL  | NULL      |

運算子 AND 和 OR 是可交換的，也就是說，您可以在不影響結果的情況下交換左右運算元。有關子表示式求值順序的更多資訊，請參閱[第 4.2.14 節](/13/the-sql-language/sql-syntax/value-expressions.md#4-2-14-biao-shi-shi-yun-suan-gui-ze)。


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