Setiap tabel turunan harus memiliki kesalahan alias sendiri

Anda memerlukan alias untuk subkueri, dan Anda perlu menerapkan ketentuan untuk kedua kueri yang Anda gabungkan:

SELECT MAX(DateTime)
FROM (

  SELECT DateTime
  FROM Class_Searches
  WHERE User_Email = '[email protected]'
  AND DateTime > NOW( ) - INTERVAL 30 DAY

  UNION ALL

  SELECT DateTime
  FROM Book_Searches
  WHERE User_Email = '[email protected]'
  AND DateTime > NOW( ) - INTERVAL 30 DAY

) AS x

atau kembalikan data sehingga Anda dapat menerapkan kondisi di kueri luar:

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 = '[email protected]'
AND DateTime > NOW( ) - INTERVAL 30 DAY