如何在PostgreSQL中使用对引用表的约束创建外键 [英] How to make a foreign key with a constraint on the referenced table in PostgreSQL

问题描述

假设我有以下表格：

CREATE TABLE plugins (
id int primary key,
type text);

insert into plugins values (1,'matrix');
insert into plugins values (2,'matrix');
insert into plugins values (3,'function');
insert into plugins values (4,'function');

CREATE TABLE matrix_params (
id int primary key,
pluginid int references plugins (id)
);

这一切都按预期工作，但我想添加一个额外的约束，matrix_param只能具有类型matrix的pluginid。

This all works as expected but I would like to add an additional constraint that a matrix_param can only refer to the pluginid that has type 'matrix'. So

insert into matrix_params values (1,1);

应该成功，但

insert into matrix_params values (2,3);

应该失败。

推荐答案

因为matrix_params不能工作，因为它无法知道插件表中对应的类型。这是一个CHECK约束。您不能将查询放在CHECK约束中，但可以调用函数;所以，我们构建一个简单的函数，告诉我们 pluginid 是一个矩阵：

You can use a CHECK constraint for this. You can't put a query in a CHECK constraint but you can call a function; so, we build a simple function that tells us if a pluginid is a matrix:

create or replace function is_matrix(int) returns boolean as $$
    select exists (
        select 1
        from plugins
        where id   = $1
          and type = 'matrix'
    );
$$ language sql;

并在CHECK约束中换行：

and wrap that in a CHECK constraint:

alter table matrix_params add constraint chk_is_matrix check (is_matrix(pluginid));

然后：

=> insert into matrix_params values (1,1);
=> insert into matrix_params values (2,3);
ERROR:  new row for relation "matrix_params" violates check constraint "chk_is_matrix"

FK负责引用完整性和级联。

And the FK takes care of referential integrity and cascades.

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