Sql 在Postgres中，如何约束两列的唯一性而忽略它们的顺序

例如，在我的Postgres数据库中，我使用两列作为主键

CREATE TABLE example (
    a integer,
    b integer,
    c integer,
    PRIMARY KEY (a, c)
);

---

我想确保不能添加

（c，a）

。换句话说，如果（1，2）已在数据库中，则无法添加（2，1）。是否可以添加这样的约束？

您可以添加基于返回的唯一索引，例如：

create unique index so45 on example ((ARRAY[greatest(a,c),least(a,c)]));

示例：

t=# insert into example select 1,2,3;
INSERT 0 1
t=# insert into example select 1,2,1;
INSERT 0 1
t=# insert into example select 3,2,1;
ERROR:  duplicate key value violates unique constraint "so45"
DETAIL:  Key ((ARRAY[GREATEST(a, c), LEAST(a, c)]))=({3,1}) already exists.

t=# alter table example drop CONSTRAINT example_pkey ;
ALTER TABLE
t=# insert into example select 1,3,3;
ERROR:  duplicate key value violates unique constraint "so45"
DETAIL:  Key ((ARRAY[GREATEST(a, c), LEAST(a, c)]))=({3,1}) already exists.

在这种情况下，不需要PK来限制唯一性：

t=# insert into example select 1,2,3;
INSERT 0 1
t=# insert into example select 1,2,1;
INSERT 0 1
t=# insert into example select 3,2,1;
ERROR:  duplicate key value violates unique constraint "so45"
DETAIL:  Key ((ARRAY[GREATEST(a, c), LEAST(a, c)]))=({3,1}) already exists.

t=# alter table example drop CONSTRAINT example_pkey ;
ALTER TABLE
t=# insert into example select 1,3,3;
ERROR:  duplicate key value violates unique constraint "so45"
DETAIL:  Key ((ARRAY[GREATEST(a, c), LEAST(a, c)]))=({3,1}) already exists.

---

您可以添加基于返回的唯一索引，例如：

create unique index so45 on example ((ARRAY[greatest(a,c),least(a,c)]));

示例：

t=# insert into example select 1,2,3;
INSERT 0 1
t=# insert into example select 1,2,1;
INSERT 0 1
t=# insert into example select 3,2,1;
ERROR:  duplicate key value violates unique constraint "so45"
DETAIL:  Key ((ARRAY[GREATEST(a, c), LEAST(a, c)]))=({3,1}) already exists.

t=# alter table example drop CONSTRAINT example_pkey ;
ALTER TABLE
t=# insert into example select 1,3,3;
ERROR:  duplicate key value violates unique constraint "so45"
DETAIL:  Key ((ARRAY[GREATEST(a, c), LEAST(a, c)]))=({3,1}) already exists.

在这种情况下，不需要PK来限制唯一性：

t=# insert into example select 1,2,3;
INSERT 0 1
t=# insert into example select 1,2,1;
INSERT 0 1
t=# insert into example select 3,2,1;
ERROR:  duplicate key value violates unique constraint "so45"
DETAIL:  Key ((ARRAY[GREATEST(a, c), LEAST(a, c)]))=({3,1}) already exists.

t=# alter table example drop CONSTRAINT example_pkey ;
ALTER TABLE
t=# insert into example select 1,3,3;
ERROR:  duplicate key value violates unique constraint "so45"
DETAIL:  Key ((ARRAY[GREATEST(a, c), LEAST(a, c)]))=({3,1}) already exists.

该模块允许将此解决方案推广到任意长度的整数集，

create extension intarray；在示例（排序（数组[a，c]）上创建唯一索引集

该模块允许将此解决方案推广到任意长度的整数集，

在阵列中创建扩展；在示例（排序（数组[a，c]）上创建唯一索引集