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