Question

I would like to know how to make a single query to select something from one table or from another. For example I have Table A with the aID=2 and Table B with bID=3. So how to make the query to return the id from table A or table B for a given id = 3?

Thanks in advance

Answer 1

If you do not know which table to use before you’re running the query, you can combine multiple queries with UNION, provided that you’re selecting the same number of columns:

SELECT something FROM A WHERE aID = 3
UNION ALL
SELECT something FROM B WHERE bID = 3

If you do not want to duplicate the WHERE condition, you can use a nested query and apply the WHERE to its result:

SELECT something
FROM (SELECT aID as ID, something FROM A
      UNION ALL
      SELECT bID      , something FROM B)
WHERE ID = 3