Slow Query Due to Sub Select - sql

Question

I have several SQL Server 2014 queries that pull back a data set where we need to get a count on related, but different criteria along with that data. We do this with a sub query, but that is slowing it down immensely. It was fine until now where we are getting more data in our database to count on. Here is the query:
SELECT
T.*,
ISNULL((SELECT COUNT(1)
FROM EventRegTix ERT, EventReg ER
WHERE ER.EventRegID = ERT.EventRegID
AND ERT.TicketID = T.TicketID
AND ER.OrderCompleteFlag = 1), 0) AS NumTicketsSold
FROM
Tickets T
WHERE
T.EventID = 12345
AND T.DeleteFlag = 0
AND T.ActiveFlag = 1
ORDER BY
T.OrderNumber ASC
I am pretty sure its mostly due to the relation back outside of the sub query to the Tickets table. If I change the T.TicketID to an actual ticket # (999 for example), the query is MUCH faster.
I have attempted to join together these queries into one, but since there are other fields in the sub query, I just cannot get it to work properly. I was playing around with
COUNT(1) OVER (PARTITION BY T.TicketID) AS NumTicketsSold
but could not figure that out either.
Any help would be much appreciated!

Answer 1

I would write this as:
SELECT T.*,
(SELECT COUNT(1)
FROM EventRegTix ERT JOIN
EventReg ER
ON ER.EventRegID = ERT.EventRegID
WHERE ERT.TicketID = T.TicketID AND ER.OrderCompleteFlag = 1
) AS NumTicketsSold
FROM Tickets T
WHERE T.EventID = 12345 AND
T.DeleteFlag = 0 AND
T.ActiveFlag = 1
ORDER BY T.OrderNumber ASC;
Proper, explicit, standard JOIN syntax does not improve performance; it is just the correct syntax. COUNT(*) cannot return NULL values, so COALESCE() or a similar function is unnecessary.
You need indexes. The obvious ones are on Tickets(EventID, DeleteFlag, ActiveFlag, OrderNumber), EventRegTix(TicketID, EventRegID), and EventReg(EventRegID, OrderCompleteFlag).

Answer 2

I would try with OUTER APPLY :
SELECT T.*, T1.*
FROM Tickets T OUTER APPLY
(SELECT COUNT(1) AS NumTicketsSold
FROM EventRegTix ERT JOIN
EventReg ER
ON ER.EventRegID = ERT.EventRegID
WHERE ERT.TicketID = T.TicketID AND ER.OrderCompleteFlag = 1
) T1
WHERE T.EventID = 12345 AND
T.DeleteFlag = 0 AND
T.ActiveFlag = 1
ORDER BY T.OrderNumber ASC;
And, obvious you need indexes Tickets(EventID, DeleteFlag, ActiveFlag, OrderNumber), EventRegTix(TicketID, EventRegID), and EventReg(EventRegID, OrderCompleteFlag) to gain the performance.

Answer 3

Fixed this - query went from 5+ seconds to 1/2 second or less. Issues were:
1) No indexes. Did not know all FK fields needed indexes as well. I indexed all the fields that we joined or were in WHERE clause.
2) Used SQL Execution Plan to see the place where the bottle neck was. Told me no index, hence 1) above! :)
Thanks for all your help guys, hopefully this post helps someone else.
Dennis
PS: Changed the syntax too!