Question

How can I be sure that my result set will have a first and b second? It would help me to solve a tricky ordering problem.

Here is a simplified example of what I’m doing:

SELECT a FROM A LIMIT 1  UNION  SELECT b FROM B LIMIT 1; 

Answer 1

SELECT col FROM     (        SELECT a col, 0 ordinal FROM A LIMIT 1        UNION ALL        SELECT b, 1 FROM B LIMIT 1    ) t ORDER BY ordinal