WHERE statement after a UNION in SQL

Question

How do I apply a WHERE statement after a UNION in SQL MySQL

User · Accepted Answer

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.

User · Answer

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

User · Answer

select column1      from table1 where column1    union select column1      from table2 where column1

[mysql] WHERE statement after a UNION in SQL?

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