Join v.s. Subquery

Question

I have a user query looks like this:
select LASTNAME,MIDDLENAME,FIRSTNAME,PHONE
from giver
where giver_nbr=
(select giver_nbr
from giver_visit
where giver_id = 1902 and visit_nbr= '0001' )
And I am wondering should this be changed to:
select a.LASTNAME,a.MIDDLENAME,a.FIRSTNAME,a.PHONE
from giver a, giver_visit b
where a.giver_nbr=b.giver_nbr
and b.giver_id = 1902
and b.visit_nbr='0001'
I looked into the explain plan and seemed to me that
the second query had a higher cost. I thought equal
join should be preferred over the subquery???
Can anybody shed some light on this? Thanks a lot!

Answer 1

1)select LASTNAME,MIDDLENAME,FIRSTNAME,PHONE
from giver
where giver_nbr=
(select giver_nbr
from giver_visit
where giver_id = 1902 and visit_nbr= '0001' )
2)select a.LASTNAME,a.MIDDLENAME,a.FIRSTNAME,a.PHONE
from giver a, giver_visit b
where a.giver_nbr=b.giver_nbr
and b.giver_id = 1902
and b.visit_nbr='0001'
In this scenario I would prefer the sub query to join for below reasons
1) You are not retrieving any info from the second table
2) The subquerys returns only one row
3) The join query has the over head of merging or NL operation which the sub query does not have