如何查找未连接的记录? [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

通常你会这样做:

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

从 a 中获取在 b 中有记录的所有记录.

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

如何只获取 a 中没有 b 中任何内容的记录?

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)

或者像这个线程上的其他人说的:

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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