每个派生表必须具有其自己的别名错误

您需要为子查询添加别名，并且需要将条件应用于您合并的两个查询：

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

或返回数据，以便您可以在外部查询中应用条件：

SELECt MAX(DateTime)FROM (  SELECt DateTime, User_Email  FROM Class_Searches  UNIOn ALL  SELECt DateTime, User_Email  FROM Book_Searches) AS xWHERe User_Email = 'bla@blah.com'AND DateTime > NOW( ) - INTERVAL 30 DAY