MySQL Forums :: PHP :: select and if statement

al salam alikum =)


guys i want to know how to fix a complicated query


i have two tables
subsystem -> 1st table
topics -> 2nd table

subsystem :
subid
userid
type
typeid ;


topics :
topicid
title
catid
userid ;


and i got 3 records ( for example ) in the subsystem table like thie
1 - 1 - user - 1
2 - 1 - user - 2
3 - 1 - cat - 1


and i wanna to fetch the topics from topics table like this

query > select * from topics inner join subsystem on topics.userid=subsystem.userid where ( and here is the problem ! i want to fetch the topics where subsystem.typeid=userid "if subsystem.type=user" and where subsystem.typeid=catid "if subsystem.type=cat")

can anybody help !

and thnx =)