如何查找未加入的记录? [英] How do I find records that are not joined?

问题描述

我有两个连接在一起的表.

I have two tables that are joined together.

A有很多B

通常您会这样做:

select * from a,b where b.a_id = a.id

要从a中具有b中记录的所有记录中获取.

To get all of the records from a that has a record in b.

我如何只获取b中没有任何内容的a中的记录?

How do I get just the records in a that does not have anything in b?

推荐答案

select * from a where id not in (select a_id from b)

或者像该线程上的其他一些人说的那样:

Or like some other people on this thread says:

select a.* from a
left outer join b on a.id = b.a_id
where b.a_id is null

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