SELECT tbl2.total_meetings AS meetings,
                               total_attendants AS attendants,
                               round(total_attendants/tbl2.total_meetings) AS average
                        FROM (
                            (SELECT SUM(tbl1.attendants) AS total_attendants
                             FROM

                                ((SELECT meeting_id AS meeting,
                                       COUNT(member_id) AS attendants
                                 FROM meeting_member
                                 GROUP BY meeting_id) AS details) tbl1 UNION

                                (SELECT COUNT(id) AS total_meetings
                                 FROM meetings) tbl2) tbl3
                        )