I get that error when running the following query:
SELECT MAX( DateTime ) FROM ( ( SELECT DateTime FROM Class_Searches ) UNION ALL ( SELECT DateTime FROM Book_Searches ) ) WHERE User_Email = 'bla@blah.com' AND DateTime > NOW( ) - INTERVAL 30 DAY I know I need to add Aliases but I'm not sure where
You need an alias for the subquery, and you need to apply the conditions either to both queries that you union:
SELECT MAX(DateTime) FROM ( SELECT DateTime FROM Class_Searches WHERE User_Email = 'bla@blah.com' AND DateTime > NOW( ) - INTERVAL 30 DAY UNION ALL SELECT DateTime FROM Book_Searches WHERE User_Email = 'bla@blah.com' AND DateTime > NOW( ) - INTERVAL 30 DAY ) AS x or return data so that you can apply the condition in the outer query:
SELECT MAX(DateTime) FROM ( SELECT DateTime, User_Email FROM Class_Searches UNION ALL SELECT DateTime, User_Email FROM Book_Searches ) AS x WHERE User_Email = 'bla@blah.com' AND DateTime > NOW( ) - INTERVAL 30 DAY An alias is when you rename something, like SELECT t.time from table t, t is the alias for that table. In this case, you need to give an alias to the tables generated by the subqueries:
SELECT MAX( ut.DateTime ) FROM ( ( SELECT DateTime FROM Class_Searches ) cs UNION ALL ( SELECT DateTime FROM Book_Searches ) bs ) ut WHERE User_Email = 'bla@blah.com' AND ut.DateTime > NOW( ) - INTERVAL 30 DAY That still won't work though, because you don't have a User_Email column returned from the UNION. Thus, try:
SELECT MAX( ut.DateTime ) FROM ( ( SELECT DateTime, User_Email FROM Class_Searches ) cs UNION ALL ( SELECT DateTime, User_Email FROM Book_Searches ) bs ) ut WHERE ut.User_Email = 'bla@blah.com' AND ut.DateTime > NOW( ) - INTERVAL 30 DAY It's possible that that still won't work right because of the UNION syntax, but at least it's a lot closer.