SQL - Selecting counts from multiple tables - sql-server

Question

Here is my problem (I'm using SQL Server)
I have a table of Students (StudentId, Firstname, Lastname, etc).
I have a table that records StudentAttendance (StudentId, ClassDate, etc.)
I record other student activity (I'm generalizing here for simplicity) such as a Papers table (StudentId, PaperId, etc.). There may be anywhere from zero to 20 papers turned in. Similarly, there is a table called Projects (StudentId, ProjectId, etc.). Same deal as with Papers.
What I'm trying to do is create a list of counts for students who have attendance over a certain level (say 10 attendances). Something like this:
ID Name Att Paper Proj
123 Baker 23 0 2
234 Charlie 26 5 3
345 Delta 13 3 0
Here is what I have:
select
s.StudentId,
s.Lastname,
COUNT(sa.StudentId) as CountofAttendance,
COUNT(p.StudentId) as CountofPapers
from Student s
inner join StudentAttendance sa on (s.StudentId = sa.StudentId)
left outer join Paper p on (s.StudentId = p.StudentId)
group by s.StudentId, s.Lastname
Having COUNT(sa.StudentId) > 10
order by CountofAttendance
If the CountofPaper and join (either inner or left outer) to the Papers table is commented out, the query works fine. I get a nice count of students who have attended at least 10 classes.
However, if I put in the CountofPapers and the join, things get crazy. With a left outer join, any students with papers just show their attendance count in the paper column. With an inner join, both attendance and paper counts seem to multiple off each other.
Guidance needed and appreciated.
Dave

Answer 1

Look at using Common Table Expressions and then divide and conquer your problem. BTW, you are off by 1 in your original query, you'll have 11 minimum attendence
;
WITH GOOD_STUDENTS AS
(
-- this query defines all students with 10+ attendance
SELECT
S.StudentID
, count(1) AS attendence_count
FROM
Student S
inner join
StudentAttendance sa
on (s.StudentId = sa.StudentId)
GROUP BY
S.StudentId
HAVING
COUNT(1) >= 10
)
, STUDIOUS_STUDENTS AS
(
-- lather, rinse, repeat for other metrics
SELECT
S.StudentID
, count(1) AS paper_count
FROM
Student S
inner join
Papers P
on (s.StudentId = P.StudentId)
GROUP BY
S.StudentId
)
, GREGARIOUS_STUDENTS AS
(
SELECT
S.StudentID
, count(1) AS project_count
FROM
Student S
inner join
Projects P
on (s.StudentId = P.StudentId)
GROUP BY
S.StudentId
)
-- And now we roll it all together
SELECT
S.*
, G.attendance_count
, SS.paper_count
, GS.project_count
-- ad nauseum
FROM
-- back to the well on this one as there may be
-- students did nothing
Students S
LEFT OUTER JOIN
GOOD_STUDENTS G
ON G.studentId = S.studentId
LEFT OUTER JOIN
STUDIOUS_STUDENTS SS
ON SS.studentId = S.studentId
LEFT OUTER JOIN
GREGARIOUS_STUDENTS GS
ON GS.studentId = S.studentId
I see plenty of other answer rolling in but I typed for far too long to quit ;)

Answer 2

The problem is there are multiple papers per student, so a StudentAttendance row for every row of Paper that joins: the counts will be re-added every time. Try this:
select
s.StudentId,
s.Lastname,
(select COUNT(*) from StudentAttendance where s.StudentId = sa.StudentId) as CountofAttendance,
(select COUNT(*) from Paper where s.StudentId = p.StudentId) as CountofPapers
from Student s
where (select COUNT(*) from StudentAttendance where s.StudentId = sa.StudentId) > 10
order by CountofAttendance
EDITED to incorporate issue with reference to CountofAttendance
btw, this isn't the fastest solution, but it is the easiest to understand, which was my intention. You can avoid the re-calculation by using a join to an aliased select, but as I said, this is the simplest.

Answer 3

Try this:
select std.StudentId, std.Lastname, att.AttCount, pap.PaperCount, prj.ProjCount
from Students std
left join
(
select StudentId, count(*) AttCount
from StudentAttendance
) att on
std.StudentId = att.StudentId
left join
(
select StudentId, count(*) PaperCount
from Papers
) pap on
std.StudentId = pap.StudentId
left join
(
select StudentId, count(*) ProjCount
from Projects
) prj on
std.StudentId = prj.StudentId
where att.AttCount > 10