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8.6. 布尔类型

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PostgreSQL提供标准的SQL类型boolean，参见表 8.19。boolean可以有多个状态：“true（真）”、“false（假）”和第三种状态“unknown（未知）”，未知状态由SQL空值表示。

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表 8.19. 布尔数据类型

\n
\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
名字存储大小说明
boolean1 字节状态为真或假
\n
\n

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在 SQL 查询中，布尔常量可以用 SQL 关键字 TRUE、FALSE 和 NULL 表示。

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boolean 类型的输入函数接受以下表示“真”状态的字符串：

\n\n\n\n\n\n\n\n\n\n\n\n\n\n
真
是
on
1
\n

以及以下表示“假”状态的字符串：

\n\n\n\n\n\n\n\n\n\n\n\n\n\n
假
否
off
0
\n

这些字符串的唯一前缀也同样可以接受，例如 t 或 n。前导或尾随空白会被忽略，并且大小写不敏感。

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boolean 类型的输出函数总是产生 t 或 f，如例 8.2所示。

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例 8.2. 使用boolean类型

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CREATE TABLE test1 (a boolean, b text);\nINSERT INTO test1 VALUES (TRUE, 'sic est');\nINSERT INTO test1 VALUES (FALSE, 'non est');\nSELECT * FROM test1;\n a |    b\n---+---------\n t | sic est\n f | non est\n\nSELECT * FROM test1 WHERE a;\n a |    b\n---+---------\n t | sic est\n
\n
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在 SQL 查询中书写布尔常量时，首选符合 SQL 标准的 TRUE 和 FALSE 关键字。也可按第 4.1.2.7 节介绍的通用字符串字面量语法使用字符串表示，例如 'yes'::boolean。

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注意，语法分析器会自动将 TRUE 和 FALSE 理解为 boolean 类型，但 NULL 不会，因为它可以是任意类型。因此在某些上下文中，你可能需要显式将 NULL 转换为 boolean，例如 NULL::boolean。反过来，如果语法分析器能够判定某个字符串字面量必定是 boolean 类型，则也可以省略这类转换。

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