选择另一个表中没有外键的主键 [英] Selecting primary keys that do not have foreign keys in another table

问题描述

为简单起见，我有两个使用外键一对多关联的表，例如:

For simplification, I have two tables related with one to many using a foreign key, for example:

Users table:
id
name

Actions table:
id
user_id

一个用户可能有很多动作或没有很多动作.我需要一个SQL select来返回在操作表中没有user_id值的用户ID.

one user may have many actions or not. I need an sql select that returns users ids that don't have a user_id value in the actions table.

Users Table:
id      name
1       John
2       Smith
3       Alice

Actions Table:
id      user_id
1       3
2       1

所以我需要一个返回用户ID 2(Smith)的SQL查询，因为外键值不包括ID 2

So I need an sql query that returns the user id 2 (Smith) because the foreign key values don't include the id 2

我尝试了以下SQL，但它返回了所有用户ID:

I tried the following SQL, but it returns all users ids:

SELECT users.id from users left join actions on actions.user_id is null

推荐答案

select u.id
from users u
left outer join actions a on a.user_id = u.id
where a.user_id is null

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