Select facid, sum(slots) as total from cd.bookings group by facid having total > 1000 order by facid;
Нельзя ссылаться на псевдоним столбца в предложении HAVING, необходимо повторить выражение:
Select facid, sum(slots) as total from cd.bookings group by facid having sum(slots) > 1000 order by facid;