I’ve coped with a problem. Let’s think we have DUMMY_TABLE. How should I choose a field from the table by id or the ‘NOT EXISTS’ line if there is no such a record? By the same time you can use only SQL, no procedure programming.
As the result I’ve got the following query:
SELECT ID from DUMMY_TABLE where id=1 UNION SELECT 'NOT EXISTS' from DUAL WHERE NOT EXISTS( select ID from DUMMY_TABLE WHERE ID=1)
It works! And how would it look like in a more elegant way?