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If you want to apply the WHERE clause to the result of the UNION, then you have to embed the UNION in the FROM clause:

SELECT *
  FROM (SELECT * FROM TableA
        UNION
        SELECT * FROM TableB
       ) AS U
 WHERE U.Col1 = ...
I'm assuming TableA and TableB are union-compatible.  You could also apply a WHERE clause to each of the individual SELECT statements in the UNION, of course.
                @Martin: it might - it might not.  You'd have to look at the query plan.  It also depends on the WHERE clause conditions.
– Jonathan Leffler
                Mar 27, 2011 at 20:43
                @Martin, Don't hope too much. Having us to do something like this shows just how dumb MySQL is....
– Pacerier
                May 5, 2015 at 8:03
You probably need to wrap the UNION in a sub-SELECT and apply the WHERE clause afterward:
SELECT * FROM (
    SELECT * FROM Table1 WHERE Field1 = Value1
    UNION
    SELECT * FROM Table2 WHERE Field1 = Value2
) AS t WHERE Field2 = Value3
Basically, the UNION is looking for two complete SELECT statements to combine, and the WHERE clause is part of the SELECT statement.
It may make more sense to apply the outer WHERE clause to both of the inner queries.  You'll probably want to benchmark the performance of both approaches and see which works better for you.
        
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