Counting records in a SQL subquery - sql

Question

I'm having difficult with a subquery. In plain English I'm trying to pick a random userID from the QCUsers table that has less than 20 records from the QCTier1_Assignments table. The problem is that my query below is only picking users where it meets the criteria of the inner query when I need it to pick any user from QCUsers table even if the user does not have any records at all in the QCTier1_Assignments table. I need something like this
AND (Sub.QCCount < 20 OR Sub.QCCount = 0 )
DECLARE #ReviewPeriodMonth varchar(10) = '10'
DECLARE #ReviewPeriodYear varchar(10) = '2015'
SELECT TOP 1
E1.UserID
,Sub.QCCount --Drawn from the subquery
FROM QCUsers E1
JOIN (SELECT
QCA.UserID,
COUNT(*) AS QCCount
FROM QCTier1_Assignments QCA
WHERE QCA.ReviewPeriodMonth = #ReviewPeriodMonth
AND QCA.ReviewPeriodYear = #ReviewPeriodYear
GROUP BY QCA.UserID
) Sub
ON E1.UserID = Sub.UserID
WHERE Active = 1
AND Grade = 12
AND Sub.QCCount < 20
ORDER BY NEWID()
I also tried it this way with no luck
DECLARE #ReviewPeriodMonth varchar(10) = '10'
DECLARE #ReviewPeriodYear varchar(10) = '2015'
SELECT TOP 1
E1.UserID
,Sub.QCCount --Drawn from the subquery
FROM QCUsers E1
RIGHT JOIN (SELECT
QCA.UserID,
ReviewPeriodMonth,
ReviewPeriodYear,
COUNT(*) AS QCCount
FROM QCTier1_Assignments QCA
GROUP BY
QCA.UserID,
ReviewPeriodMonth,
ReviewPeriodYear
) Sub
ON E1.UserID = Sub.UserID
WHERE Active = 1
AND Grade = 12
AND Sub.QCCount < 20
AND Sub.ReviewPeriodMonth = #ReviewPeriodMonth
AND Sub.ReviewPeriodYear = #ReviewPeriodYear
ORDER BY NEWID()

Answer 1

Try using your second query but change the WHERE clause to use COALESCE(Sub.QCCount, 0) instead of justSub.QCCount`
If the subquery returns no rows then with your RIGHT JOIN you'll at least still get the row, but the QCCount will be NULL which when compared to anything will result in a "false" effectively.
Also, you should look into the HAVING clause. It might allow you to do this without a subquery at all.
Here's an example with the HAVING clause. If it doesn't give the correct results please let me know as I'm not able to test this.
DECLARE
#ReviewPeriodMonth VARCHAR(10) = '10'
#ReviewPeriodYear VARCHAR(10) = '2015'
SELECT TOP 1
E1.UserID,
COUNT(QCA.UserID) AS QCCount
FROM
QCUsers E1
LEFT OUTER JOIN QCTier1_Assignments QCA ON
QCA.UserID = E1.UserID AND
QCA.ReviewPeriodMonth = #ReviewPeriodMonth AND
QCA.ReviewPeriodYear = #ReviewPeriodYear
WHERE
E1.Active = 1 AND
Grade = 12 AND
HAVING
COUNT(*) < 20
ORDER BY
NEWID()

Answer 2

You should use LEFT JOIN instead of JOIN(INNER JOIN), And you'd better to put the predicate to the outer query based on your practice, but I recommend the following way:
SELECT TOP1 ABC.UserID,ABC.QCCount
FROM
(
SELECT E1.UserID, COUNT(*) as QCCount
FROM QCUsers as E1
LEFT JOIN QCTier1_Assignments as QCA
ON QCA.UserID = E1.UserID
WHERE QCA.ReviewPeriodMonth = #ReviewPeriodMonth
AND QCA.ReviewPeriodYear = #ReviewPeriodYear
AND Active = 1
AND Grade = 12
GROUP BY E1.UserID
) as ABC
WHERE ABC.QCCount <20
ORDER BY NEWID()

Answer 3

I was able to work it out through a combination of responses here
DECLARE #ReviewPeriodMonth varchar(10) = '10'
DECLARE #ReviewPeriodYear varchar(10) = '2015'
SELECT TOP 1
QCUsers.UserID,
COUNT(QCTier1_Assignments.ReviewID) AS ReviewCount
FROM
QCTier1_Assignments RIGHT OUTER JOIN
QCUsers ON QCTier1_Assignments.UserID = QCUsers.UserID
WHERE
QCUsers.Active = 1
AND QCUsers.Grade = '12'
AND (ReviewPeriodMonth = #ReviewPeriodMonth OR ReviewPeriodMonth IS NULL)
AND (ReviewPeriodYear = #ReviewPeriodYear OR ReviewPeriodYear IS NULL)
GROUP BY
QCUsers.UserID
HAVING
(COALESCE(COUNT(QCTier1_Assignments.ReviewID),0) < 4)
ORDER BY NEWID()