1. 定义enum: (男、女)

postgres=# create type sex as enum('male','female');
CREATE TYPE

postgres=# create table person(id serial,name character varying,p_sex sex);
CREATE TABLE

postgres=# \d person
               Table "public.person"
Column |    Type    |           Modifiers           
--------+-------------------+-----------------------------------------------------
id   | integer      | not null default nextval('person_id_seq'::regclass)
name  | character varying |
p_sex | sex        |

postgres=# insert into person(name,p_sex) values('zhangsan','aa');
<span style="color:#ff0000;">ERROR:  invalid input value for enum sex: "aa"</span>
LINE 1: insert into person(name,'aa');
                                                         ^
postgres=# insert into person(name,'male');
INSERT 0 1

postgres=# select * from person;
 id |   name   | p_sex 
----+----------+-------
  1 | zhangsan | male
(1 row)

可以看到，枚举类型的字段在使用不是枚举里面定义的值时候会报错，相当于一个check。

4. 系统表 pg_enum:

我们定义的所有枚举在系统表pg_enum中可以看到，类似实际应用中的‘字段表’。

postgres=# create type people as enum('child','young','old');
CREATE TYPE

postgres=# select * from pg_enum ;
 enumtypid | enumsortorder | enumlabel 
-----------+---------------+-----------
     98345 |             1 | male
     98345 |             2 | female
     98359 |             1 | child
     98359 |             2 | young
     98359 |             3 | old
(5 rows)