SQL - If Select returns nothing then do another Select

Question

I'm currently trying to do an SQL query that can detect if a SELECT query returns nothing, and then do another one if that is the case.

Here is what I mean:

IF SELECT * FROM table WHERE criteria = criteria RETURNS NO ROWS
THEN SELECT * FROM table WHERE criteria2 = criteria2

Is this possible? I don't think that an empty reply counts as "null" so I have a bit of a trouble with that.

Answer 1

You can do this in one statement, assuming the columns are the same:

SELECT *
FROM table
WHERE criteria = criteria 
UNION ALL
SELECT *
FROM table
WHERE criteria2 = criteria2 AND
      NOT EXISTS (SELECT 1 FROM table WHERE criteria = criteria);