Question

我有一个简单的数据库：

CREATE TABLE workpiece
(
  id serial primary key
  -- More columns, not important here
);

CREATE TABLE workequipment
(
  id serial primary key
  -- More columns, not important here
);

CREATE TABLE workpiece_workequipment
(
  workpiece_id integer not null references workpiece(id),
  workequipment_id integer not null references workequipment(id),
  primary key(workpiece_id, workequipment_id)
);

但现在需求发生变化：我必须包含类型 workequipment（如工具，测量设备等）也是如此并强制我们不能在每个工件上多次使用相同类型的工件设备。

所以我想出了以下内容：

CREATE TABLE workpiece
(
  id serial primary key
  -- More columns, not important here
);

CREATE TABLE workequipment
(
  id serial primary key,
  equipment_type integer, -- An enum in real world
  -- More columns, not important here
  unique(id, equipment_type)
);

CREATE TABLE workpiece_workequipment
(
  workpiece_id integer not null references workpiece(id),
  workequipment_id integer not null,
  workequipment_type integer not null,
  primary key(workpiece_id, workequipment_id),
  foreign key(workequipment_id, workequipment_type) references workequipment(id, equipment_type),
  unique(workpiece_id, workequipment_type)
);

使用这种冗余来强制执行约束是否可以？如果是，我应该在表workequipment中删除唯一（id，equipment_type）而使（id，equipment_type）成为主键？或者是否有更好的解决方案？

Answer 1

您需要功能唯一索引：

CREATE TABLE workpiece
(
  id serial primary key
  -- More columns, not important here
);

CREATE TABLE workequipment
(
  id serial primary key,
  equipment_type integer
  -- More columns, not important here
);

CREATE TABLE workpiece_workequipment
(
  workpiece_id integer not null references workpiece(id),
  workequipment_id integer not null references workequipment(id),
  primary key(workpiece_id, workequipment_id)
);

-- Magic starts here :)

create function get_workequipment_type(int) returns int immutable language sql as $$
  select equipment_type from workequipment where id = $1
$$;

create unique index idx_check_wetype_unique
  on workpiece_workequipment(workpiece_id, get_workequipment_type(workequipment_id));

测试：

insert into workpiece values(default);
insert into workequipment(equipment_type) values(1),(1),(2);
insert into workpiece_workequipment values(1,1),(1,3); -- Works
--insert into workpiece_workequipment values(1,1),(1,2); -- Fails

Answer 2

这非常难看，但我也无法提出更好的解决方案。

您必须同时保留workequipment上的主键和唯一约束，否则您最终可能会遇到重复的id。

SQL约束取决于几个表

2 个答案: