COUNT of different tables in GROUP - sql

Question

I have a table that holds a list of classes available at a school. Each class can have a number of sessions. And each class can have pupils assigned to it.
What I need to do is get a count of all sessions for each class, as well as the number of students attending the class. I have done the first bit, but if I join to the pupil allocation table, my counts will be wrong.
I have conjured up some fake SQL that you can use.
I'm stuck with efficiently getting a count from the Pupil link table.
DECLARE #Class TABLE
(
ClassID INT NOT NULL,
ClassName VARCHAR(20) NOT NULL
)
INSERT INTO #Class VALUES (1, 'English')
INSERT INTO #Class VALUES (2, 'Maths')
DECLARE #ClassSession TABLE
(
ClassSessionID INT NOT NULL,
ClassID INT NOT NULL,
Description VARCHAR(100) NOT NULL
)
INSERT INTO #ClassSession VALUES (1, 1, 'Basic English')
INSERT INTO #ClassSession VALUES (2, 1, 'Advanced English')
INSERT INTO #ClassSession VALUES (3, 1, 'Amazing English')
INSERT INTO #ClassSession VALUES (4, 2, 'Basic English')
INSERT INTO #ClassSession VALUES (5, 2, 'Basic English')
DECLARE #ClassPupil TABLE
(
ClassPupilID INT NOT NULL,
ClassID INT NOT NULL,
PupilID INT NOT NULL -- FK to the Pupils table.
)
INSERT INTO #ClassPupil VALUES (1, 1, 1000)
INSERT INTO #ClassPupil VALUES (2, 1, 1001)
INSERT INTO #ClassPupil VALUES (3, 1, 1002)
INSERT INTO #ClassPupil VALUES (4, 1, 1003)
INSERT INTO #ClassPupil VALUES (5, 1, 1004)
INSERT INTO #ClassPupil VALUES (6, 2, 1005)
INSERT INTO #ClassPupil VALUES (7, 2, 1006)
INSERT INTO #ClassPupil VALUES (8, 2, 1007)
SELECT ClassName, COUNT(*) AS Sessions, '??' AS NumerOfPupils
FROM #Class c
INNER JOIN #ClassSession cs
ON cs.ClassID = c.ClassID
GROUP BY c.ClassID, c.ClassName
It can maybe be done with a sub query? Is that the best way?

Answer 1

You have two independent dimensions for each class. You need to aggregat them separately:
SELECT c.ClassName, cs.Sessions, cp.Pupils
FROM #Class c INNER JOIN
(SELECT ClassId, COUNT(*) as sessions
FROM #ClassSession cs
GROUP BY ClassId
) cs
ON cs.ClassID = c.ClassID INNER JOIN
(SELECT ClassId, COUNT(*) as pupils
FROM #ClassPupil cp
GROUP BY ClassId
) cp
ON cp.ClassId = c.ClassId;

Answer 2

Another method is to use CROSS APPLY to get the count of pupils:
SELECT
ClassName, COUNT(*) AS Sessions, cp.NumberOfPupils
FROM #Class c
INNER JOIN #ClassSession cs
ON cs.ClassID = c.ClassID
CROSS APPLY (
SELECT COUNT(*) AS NumberOfPupils
FROM #ClassPupil
WHERE
ClassID = c.ClassID
) cp
GROUP BY c.ClassID, c.ClassName, cp.NumberOfPupils

Answer 3

SELECT ClassName, COUNT(distinct cs.ClassSessionID) AS Sessions, /*'??'*/ count( distinct cp.PupilID) AS NumerOfPupils
FROM #Class c
INNER JOIN #ClassSession cs
ON cs.ClassID = c.ClassID
inner join #ClassPupil cp on c.ClassID=cp.ClassID
GROUP BY /*c.ClassID,*/ c.ClassName
count(distinct...) solves (works around) the problem.
Generally, this is (A -> B, C) issue.