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
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