理解多个连接的问题

Question

我有以下表格：

users
 ____________
| id         |
| first_name |
|____________|

pre_task_plans
 ____________
| id         |
| creator_id | (fk user_id)
|____________|

pre_task_plan_users
 __________________
| id               |
| user_id          |
| pre_task_plan_id |
|__________________|

我正在尝试让所有用户获得pre_task_plans，我是（users.id 1）的创建者

我有

SELECT users.*
FROM users
JOIN pre_task_plans as ptp on ptp.creator_id = 1
JOIN pre_task_plan_users as ptpu on ptpu.pre_task_plan_id = ptp.id

但它返回的用户不在与pre_task_plans相关的pre_task_plan_users中。我不了解第二次JOIN是如何工作的？

PSQL 9.6.2

Answer 1

SELECT users.*, other.*
FROM users
JOIN pre_task_plans as ptp on ptp.creator_id = users.id
JOIN pre_task_plan_users as ptpu on ptpu.pre_task_plan_id = ptp.id
join users as other on pre_task_plan_users.user_id = other.id
where users.id = 1

您将每个pre_task_plan加入其创建的user，然后将结果缩小为您的用户（仅限您创建的那些）。

您也应该显示pre_task_plans中的一些行，这样您只能看到自己的id和first_name