Every derived table must have its own alias error

Question

I get that error when running the following query:

SELECT MAX( DateTime )
FROM (
(
    SELECT DateTime
    FROM Class_Searches
)
UNION ALL (
    SELECT DateT         


        

Answer 1

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.