Alternative/enhancement to union query? Duplicate - please delete

Question

Hello,
I have 2 sets of similar data which I am joining together with a union query.
Simplified E.g.
*Table A*
Name Dept
Ben    1
Amy   3
Ken    3
*Table B*
Name Dept
Jim    1
John  2
Sue   3
select Name, Dept
from table A
UNION
select Name, Dept
from table BWhat I would like to do is ONLY include data from the second part of the union query where it joins with some data from the first part of the union query.
E.g.
select Name, Dept
from table A
UNION
select Name, Dept
from table B
where Dept in (select distinct Dept from A)Therefore John that works in Dept 2 would not be included in the results.
Is there a better or more efficient way of doing this than my example using the subquery as part of the where clause?
Many thanks,
Hazel
Edited by: user11979866 on 04-Nov-2010 08:48

Answer 1

Hazel:
It would really depend on the sizes of the tables and the indexes available, but that is probably the way I would write it, at least as a first cut.
You could lose the distinct in the in sub-query since it is implied by the in itself, and Oracle will only return one row from the sub-query for each value of dept in A.
John