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

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