Filter union result

Question

I'm making select with union. select * from table_1 union select * from table_2 ...

Is it possible to filter query result by column values?

Answer 1

Yes, you can enclose your entire union inside another select:

select * from (
select * from table_1 union select * from table_2) as t
where t.column = 'y'

You have to introduce the alias for the table ("as t"). Also, if the data from the tables is disjoint, you might want to consider switching to UNION ALL - UNION by itself works to eliminate duplicates in the result set. This is frequently not necessary.