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

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.