Postgresql 基于多表跨多表计数

Question

我有 4 个表，目标是一个简单的真/假（或者如果更容易计数）是否存在基于给定的 User.uuid 和 Resource.uuid 的 AccountLink 或 ResourceLink。

最小的工作示例

http://sqlfiddle.com/#!17/09144/1

目标

基于User.uuid 和Resource.uuid 的正确/错误响应，它们不相关。

架构

User
- id
- uuid

Resource
- id
- account_id
- uuid

AccountLink
- id
- account_id
- user_id

ResourceLink
- id
- resource_id
- user_id

目前工作中

给出id列表而不是true/false，可以在应用程序代码中处理，但是查询本身也是低效的：

SELECT id
FROM "AccountLinks"
WHERE user_id = (SELECT id FROM "Users" WHERE uuid = ?) 
  AND account_id = (SELECT account_id FROM "Resources" WHERE uuid = ?)
UNION
SELECT id
FROM "ResourceLinks"
WHERE user_id = (SELECT id FROM "Users" WHERE uuid = ?)
  AND resource_id = (SELECT id FROM "Resources" WHERE uuid = ?)

有没有办法根据uuid 找到正确的user 和resource 一次，然后使用这些id 属性对多个表进行计数？

Answer 1

嗯。 . .我想你想要一个 cross join 并提供额外的信息：

select 
  (exists (select 1
     from "AccountLinks" al
     where al.user_id = u.id and al.account_id = r.account_id
  )) as has_accountlink,
  (exists (select 1
     from "ResourceLinks" rl
     where rl.user_id = u.id and rl.resource_id = r.id
  )) as has_resourcelink
from "Users" u cross join
     "Resources" r 
where u.uuid = ? and r.uuid = ?