Question

I have a complex nested-query which is inside a join, is it possible to find several columns that match that query instead of repeating the query in the Join? ie:

select * from 
A left join B on a.xid=b.xid and 
(a.userid or b.userid) in (select userid from A where..) 

^^^ don’t want to duplicate the nested-query…

There is a nested query that should match several columns from the parent-query (as seen in the example above). The simple way is to duplicate the nested query several times. ie-

select * from A
left join B
  on a.xid=b.xid
  and a.userid in (select userid from ...)
  and b.userid in (Select userid from ....)

BUT – since the subquery is bit complicated I don’t want mysql to run it twice, but rather only once and than match it against several of the parent query columns.

Answer 1

If your subquery is working properly and you have the query cache turned on you won’t have to worry about performance. If its a question of it being overly complex then maybe you could use a proc for this query: put the results of the sub into a temp table and join to it.

There are lots of ways to approach this.