Question

I’m a beginner at SQL and have this fairly easy conditional problem: Every installation number in the database has a customer. But I have been told that the customer is in either the AUDEB table or the AFORD table. I should first look in AUDEB for CUSTOMER_NO and use that if it is not NULL. If it is NULL, then take the CUSTOMER_NO from the AFORD table.

Use this if CUSTOMER_NO is not NULL

  SELECT CUSTOMER_NO
  FROM AUDEB
  WHERE INST_NO = 2

Else use this CUSTOMER_NO

  SELECT CUSTOMER_NO
  FROM AFORD
  WHERE INST_NO = 2

I see that there exist IF…ELSE condtions in SQL, but isn’t there an easier way of selecting between the values from two queries where I want to use the first if the result is not null, else use the other?

Answer 1

You could union the tables using a subquery to retrieve a complete list of customers:

select  CUSTOMER_NO
from    (
        select  CUSTOMER_NO
        ,       INST_NO
        from    AUDEB
        union all
        select  CUSTOMER_NO
        ,       INST_NO
        from    AFORD
        ) as all_customers
where   INST_NO = 2