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8.7. 枚举类型

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枚举（enum）类型是由一个静态的有序值集合构成的数据类型。它们等效于很多编程语言所支持的enum类型。枚举类型的一个示例可以是一周中的各天，或者一个数据的状态值集合。

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8.7.1. 枚举类型的声明

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可以使用CREATE TYPE命令创建枚举类型，例如：

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CREATE TYPE mood AS ENUM ('sad', 'ok', 'happy');\n
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创建之后，该 enum 类型就可以像任何其他类型一样在表定义和函数定义中使用：

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CREATE TYPE mood AS ENUM ('sad', 'ok', 'happy');\nCREATE TABLE person (\n    name text,\n    current_mood mood\n);\nINSERT INTO person VALUES ('Moe', 'happy');\nSELECT * FROM person WHERE current_mood = 'happy';\n name | current_mood\n------+--------------\n Moe  | happy\n(1 row)\n
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8.7.2. 排序

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enum 类型中各值的顺序，就是创建该类型时列出的顺序。枚举支持所有标准比较操作符以及相关聚合函数。例如：

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INSERT INTO person VALUES ('Larry', 'sad');\nINSERT INTO person VALUES ('Curly', 'ok');\nSELECT * FROM person WHERE current_mood > 'sad';\n name  | current_mood\n-------+--------------\n Moe   | happy\n Curly | ok\n(2 rows)\n\nSELECT * FROM person WHERE current_mood > 'sad' ORDER BY current_mood;\n name  | current_mood\n-------+--------------\n Curly | ok\n Moe   | happy\n(2 rows)\n\nSELECT name\nFROM person\nWHERE current_mood = (SELECT MIN(current_mood) FROM person);\n name\n-------\n Larry\n(1 row)\n
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8.7.3. 类型安全性

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每一种枚举数据类型都是独立的并且不能和其他枚举类型相比较。看这样一个示例：

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CREATE TYPE happiness AS ENUM ('happy', 'very happy', 'ecstatic');\nCREATE TABLE holidays (\n    num_weeks integer,\n    happiness happiness\n);\nINSERT INTO holidays(num_weeks,happiness) VALUES (4, 'happy');\nINSERT INTO holidays(num_weeks,happiness) VALUES (6, 'very happy');\nINSERT INTO holidays(num_weeks,happiness) VALUES (8, 'ecstatic');\nINSERT INTO holidays(num_weeks,happiness) VALUES (2, 'sad');\nERROR:  invalid input value for enum happiness: \"sad\"\nSELECT person.name, holidays.num_weeks FROM person, holidays\n  WHERE person.current_mood = holidays.happiness;\nERROR:  operator does not exist: mood = happiness\n
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如果你确实需要做这类事情，可以编写自定义操作符，或者在查询中添加显式类型转换：

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SELECT person.name, holidays.num_weeks FROM person, holidays\n  WHERE person.current_mood::text = holidays.happiness::text;\n name | num_weeks\n------+-----------\n Moe  |         4\n(1 row)\n\n
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8.7.4. 实现细节

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枚举标签是大小写敏感的，因此'happy'与'HAPPY'是不同的。标签中的空白也是有意义的。

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尽管枚举类型的主要目的是用于值的静态集合，但也有方法在现有枚举类型中增加新值和重命名值（见ALTER TYPE）。不能从枚举类型中去除现有的值，也不能更改这些值的排序顺序；除非删除并重建枚举类型，否则无法这样做。

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枚举值在磁盘上占四字节。枚举值的文本标签长度受编译 PostgreSQL 时设置的 NAMEDATALEN 限制；在标准构建中，最多为 63 字节。

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从内部枚举值到文本标签的转换关系被保存在系统目录pg_enum中。直接查询该目录可能会有所帮助。

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