SQL & PL/SQL

Hi

I have two queries

Query 1:-

SELECT DTN
      FROM tabA
    WHERE INSTANCE_ID = 123 AND PROCESS_ID = p123

Query:-2

SELECT DTN
          FROM tabB
    WHERE INSTANCE_ID = 123 AND PROCESS_ID = p123

Whenever no data found on result set of query1 then query2 to executed and vice-versa.

How it can be achieved in a single query..?

Thanks
Raj