Question

I want to SELECT values based on a column called LOCATION, but I want to SELECT values if LOCATION has values for ‘TOWN’ if there are no values for LOCATION ='TOWN' then SELECT values for LOCATION='CITY'.

There are always values in the table for LOCATION='CITY' but we want to get values from the table for LOCATION='TOWN' and if this returns no values then fall back to LOCATION='CITY'.

Can this be done in the same (single) SELECT.

I don’t think this can be accomplished using a simple DECODE.

Answer 1

This seems better suited to a function, but it is possible (at least, based on my understanding of your question).

I’ve created a sql fiddle to demo it.

Replace City/Town with your values or IDs. The EXISTS clause searches for your first level search (city, based on your question) – if that fails, it falls back to matching against the else condition (town, in this case).

SELECT id, population
  FROM locations
 WHERE
  CASE WHEN EXISTS
      (
        SELECT id
        FROM locations l2 
        WHERE location = 'City'
      ) THEN 'City'
  ELSE 
      'Town'
  END = location