CODEWITHSUNDEEP
mysqlsql
einstein
einstein

Reputation: 13850

WHERE statement after a UNION in SQL?

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

Upvotes: 100

Views: 171033

Answers (3)

pinkykitty
pinkykitty

Reputation: 35

select column1..... from table1
where column1=''
union
select column1..... from table2
where column1= ''

Upvotes: 0

David
David

Reputation: 218808

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.

Upvotes: 28

Jonathan Leffler
Jonathan Leffler

Reputation: 753525

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.

Upvotes: 171

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