subquery using IN clause

Question

----------
User
----------
user_ID(pk)
UserEmail

----------
Employer1
----------
Emp1ID(pk)
Emp1NO

----------
Employer2
----------
Emp2ID(pk)
Emp2NO

----------
Pr         


        

Answer 1

Eric Hauser is correct - you can't specify two columns in SELECT contained in an IN clause to one column outside.

That said, I re-wrote your query so you don't need to use UNIONs at all:

SELECT DISTINCT u.user_email
  FROM USERS u
  JOIN EMPLOYER e ON u.user_id IN (e.emp1id, e.emp2id) 
  JOIN PROJECT p ON e.emp1no IN (p.projempid, p.projempmgrid)
                 OR e.emp2no IN (p.projempid, p.projempmgrid)

I also changed the query to use ANSI-92 JOIN syntax - your original uses ANSI-89.