>>Do select on FORCED and if it returns 0 record, run your regular select.
>>
>
>Is there a way to do this logic in one select?

try ( not verified )

SELECT
NVL(X.FIELD1,Y.FIELD1) FIELD1
....
FROM  TABLE1 X FULL JOIN TABLE1 Y ON ISNULL(X.PK)
 WHERE X.FORCEDCONDITION AND Y.REGULARCONDITION

Of course, it is not efficient with a lot of rows