2019独角兽企业重金招聘Python工程师标准>>> 
enum的使用
db01=> create type enum_clor as enum ('red','black','green','white','blue');
CREATE TYPE
db01=> create table t_pencil(id int,clor enum_clor);
CREATE TABLE
db01=> insert into t_pencil values(1,'red');
INSERT 0 1
db01=> insert into t_pencil values(1,'yellow');
ERROR: invalid input value for enum enum_clor: "yellow"
LINE 1: insert into t_pencil values(1,'yellow');
^
db01=> select * from t_pencil;
1 | red
enum是一种有顺序的集合(set).
我们测试下排序
db01=> insert into t_pencil values(1,'red');
INSERT 0 1
db01=> insert into t_pencil values(2,'white');
INSERT 0 1
db01=> insert into t_pencil values(3,'blue');
INSERT 0 1
db01=> insert into t_pencil values(4,'black');
INSERT 0 1
db01=> insert into t_pencil values(5,'green');
INSERT 0 1
db01=> select * from t_pencil order by clor;
1 | red
4 | black
5 | green
2 | white
3 | blue
enum的顺序就是创建type时指定的,而非按照字符串或数字大小排
enum 类型安全
我们再建个enum并建一个表
db01=> create type enum_clor1 as enum ('red','black','green','white','blue');
CREATE TYPE
db01=> create table t_paper(id int,clor enum_clor1);
CREATE TABLE
db01=> insert into t_paper values(1,'red');
INSERT 0 1
db01=> insert into t_paper values(2,'blue');
INSERT 0 1
db01=> select * from t_paper;
1 | red
2 | blue
测试
db01=> select * from t_pencil c,t_paper p where c.clor = p.clor;
ERROR: operator does not exist: enum_clor = enum_clor1
LINE 1: select * from t_pencil c,t_paper p where c.clor = p.clor;
^
HINT: No operator matches the given name and argument type(s). You might need to add explicit type casts.
db01=> select * from t_pencil c,t_paper p where c.clor::text = p.clor::text;
3 | blue | 2 | blue
1 | red | 1 | red
不同enum类型是不能直接比较的,即使字符串一样.除非显式转换或者自定义操作符.
enum相关函数
db01=> select enum_first('black'::enum_clor),enum_first(null::enum_clor);
red | red
db01=> select enum_last('black'::enum_clor),enum_last(null::enum_clor);
blue | blue
db01=> select enum_range('black'::enum_clor),enum_range(null::enum_clor);
{red,black,green,white,blue} | {red,black,green,white,blue}
两个参数的enum_range
db01=> select enum_range(null,'white'::enum_clor);
{red,black,green,white}
db01=> select enum_range('black'::enum_clor,null);
{black,green,white,blue}
db01=> select enum_range('black'::enum_clor,'white'::enum_clor);
{black,green,white}
enum_first返回enum的第一个枚举值
enum_last返回enum的最后一个枚举值
enum_range返回enum的所有枚举值数组按顺序排列
enum_range 两个入参的,则返回符合条件的枚举类型数组. 除了两个参数enum_range,其他几个函数的入参忽略具体的枚举值,只关心枚举类型
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