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8.10. 位串类型

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位串是由 1 和 0 组成的字符串。它们可用于存储或展示位掩码。SQL 中有两种位类型：bit(n) 和 bit varying(n)，其中 n 是正整数。

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bit 类型的数据长度必须与 n 完全一致；试图存储更短或更长的位串都会报错。bit varying 数据则是长度上限为 n 的变长类型，更长的串会被拒绝。不带长度的 bit 等效于 bit(1)，而不带长度说明的 bit varying 表示长度不受限。

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注意

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如果显式地将一个位串值转换为 bit(n)，它会在右侧被截断或补零，直到长度恰好为 n 位，且不会报错。类似地，如果显式地将一个位串值转换为 bit varying(n)，而其长度超过 n 位，则会在右侧被截断。

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关于位串常量的语法信息，请参见第 4.1.2.5 节。位逻辑操作符和字符串操作函数也可用；参见第 9.6 节。

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例 8.3. 使用位串类型

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CREATE TABLE test (a BIT(3), b BIT VARYING(5));\nINSERT INTO test VALUES (B'101', B'00');\nINSERT INTO test VALUES (B'10', B'101');\n\nERROR:  bit string length 2 does not match type bit(3)\n\nINSERT INTO test VALUES (B'10'::bit(3), B'101');\nSELECT * FROM test;\n\n  a  |  b\n-----+-----\n 101 | 00\n 100 | 101\n\n
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位串值每 8 位需要 1 字节，另有 5 或 8 字节的开销，具体取决于字符串的长度（但长值可能被压缩或移到行外存储，参见第 8.3 节对字符串的说明）。

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