How do I find records that are not joined?

Question

I have two tables that are joined together.

A has many B

Normally you would do:

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

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

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

Answer 1

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

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