Get the average attendance? - subquery

Question

I am trying to develop a subquery to get the average attendance from two tables along with full details

Answer 1

select EVENTID ,
EVENTNAME ,
STARTDATETIME ,
ENDDATETIME ,
CONCERT_CONCERTID ,
VENUE_VENUEID,COUNT(TIMEOFARRIVAL),AVG(TIMEOFARRIVAL)
FROM EVENT INNER JOIN BOOKING ON BOOKING.EVENT_EVENTID = EVENT.EVENTID;
There is an error inconsistent datatypes expected number got timestamp

Answer 2

Your first step is to get the number of attendees per event:
SELECT e.eventid, COUNT(b.customer_customerid) AS customer_cnt
FROM event e LEFT JOIN booking b
ON e.eventid = b.event_eventid
GROUP BY e.eventid;
Now I use a LEFT JOIN above because an event could conceivably have zero attendance and those should be counted as well!
We can get the average using a window function (what Oracle calls an analytic function):
SELECT eventid, attendance, AVG(attendance) OVER ( ) AS avg_attendance
FROM (
SELECT e.eventid, COUNT(b.customer_customerid) AS attendance
FROM event e LEFT JOIN booking b
ON e.eventid = b.event_eventid
GROUP BY e.eventid
);
Now we can get all events with below-average attendance:
SELECT eventid, attendance FROM (
SELECT eventid, attendance, AVG(attendance) OVER ( ) AS avg_attendance
FROM (
SELECT e.eventid, COUNT(b.customer_customerid) AS attendance
FROM event e LEFT JOIN booking b
ON e.eventid = b.event_eventid
GROUP BY e.eventid
)
) WHERE attendance < avg_attendance;
The above will give you a list of eventids - you can get the rest of the columns you need by querying the event table with this list.
Edited based on comments - poster would like to determine the events for each concert with below average attendance for that concert.
SELECT concertid, eventid, attendance, avg_attendance FROM (
SELECT concertid, eventid, attendance, AVG(attendance) OVER ( PARTITION BY concertid ) AS avg_attendance
FROM (
SELECT c.concertid, e.eventid, COUNT(b.timeofarrival) AS attendance
FROM concert c INNER JOIN event e
ON c.concertid = e.concert_concertid
LEFT JOIN booking b
ON e.eventid = b.event_eventid
GROUP BY e.eventid
)
) WHERE attendance < avg_attendance;