find parent rows not having a child

1.select * from parent where not exists(select 1 from child where parent.pk=child.fk)
this is better since no join is involved

2.select * from parent where parent.pk not in (select fk from child)
this is most natural query. a little bit worse than first method since join operation is heavier

3.out join
select * from parent left (outer) join child on parent.pk=child.fk where child.fk is null

this utilize the feature of outer join. you make a result from join the first, then filter out the record wanted. might be worst performance.