SQL- Subquery with aggregate functions (SQL Server) - sql-server

Question

My set up is as thus: I have three tables-
Students (StudentID, FirstName, LastName etc.),
StudentSemesters(StudentID,SemID etc.), and
Semesters(SemID, Year)
My requirement is to, get the details for each student but only for their last semester. Logically this implies the semester with the highest year number. I cannot seem to get the Query right. For simplicity 'Year' is just an integer (e.g. 2000, 1998). Below is the current query that I have been stuck at for some time:
SELECT dbo.Student.LastName + ' , ' + dbo.Student.FirstName AS Student, dbo.Student.Defence1Date, dbo.Student.Defence2Date, COUNT(StudentSemesters_1.SemID)
AS SemesterCount, dbo.Student.EntrySemester + ' - ' +
(SELECT dbo.StudentSemesters.SemID
FROM dbo.StudentSemesters INNER JOIN
dbo.ListSemesters ON dbo.StudentSemesters.SemID = dbo.ListSemesters.SemID
WHERE (dbo.Student.StudentCode = dbo.StudentSemesters.StudentCode)
GROUP BY dbo.StudentSemesters.SemID, dbo.ListSemesters.Year
HAVING (dbo.ListSemesters.Year = MAX(dbo.ListSemesters.Year))) AS Expr1
FROM dbo.Student INNER JOIN
dbo.StudentSemesters AS StudentSemesters_1 ON dbo.Student.StudentCode = StudentSemesters_1.StudentCode
GROUP BY dbo.Student.LastName, dbo.Student.FirstName, dbo.Student.Defence1Date, dbo.Student.Defence2Date, dbo.Student.EntrySemester,
dbo.Student.StudentCode

Answer 1

You can do this to get the details for only the last semester for each student:
SELECT
s.LastName + ' , ' + s.FirstName AS Student,
s.Defence1Date,
s.EntrySemester + ' - ' + s1.SemId AS Expr1
FROM dbo.Student AS s
INNER JOIN dbo.StudentSemesters AS S1 ON s.StudentCode = S1.StudentCode
INNER JOIN dbo.ListSemesters AS lm ON lm.SemID = s1.SemId
INNER JOIN
(
SELECT SemId, MAX(Year) AS MaxYear
FROM dbo.StudentSemesters
GROUP BY SemId
) AS s2 ON s2.semId = lm.SemId AND ls.Year = s2.MaxYear;
However, if you're using SQL Server 2005+, you can use the window function to do so:
WITH CTE
AS
(
SELECT
s.LastName + ' , ' + s.FirstName AS Student,
s.Defence1Date,
s.EntrySemester + ' - ' + s1.SemId AS Expr1,
ROW_NUMBER() OVER(PARTITION BY s1.SemId
ORDER BY Year DESC) AS RowNumber
FROM dbo.Student AS s
INNER JOIN dbo.StudentSemesters AS S1 ON s.StudentCode = S1.StudentCode
INNER JOIN dbo.ListSemesters AS lm ON lm.SemID = s1.SemId
)
SELECT
Student,
Defence1Date,
Expr1,
FROM CTE
WHERE RN = 1;
But this won't get the SemesterCount, but you might be able to make it like this (this is just a guess):
SELECT
s.LastName + ' , ' + s.FirstName AS Student,
s.Defence1Date,
s1.SemCount,
s.EntrySemester + ' - ' + s1.SemId AS Expr1
FROM dbo.Student AS s
INNER JOIN
(
SELECT StudentCode, COUNT(SemId) AS SemCount
FROM dbo.StudentSemesters
GROUP BY StudentCode
) AS S1 ON s.StudentCode = S1.StudentCode
INNER JOIN dbo.ListSemesters AS lm ON lm.SemID = s1.SemId
INNER JOIN
(
SELECT SemId, MAX(Year) AS MaxYear
FROM dbo.StudentSemesters
GROUP BY SemId
) AS s2 ON s2.semId = lm.SemId AND ls.Year = s2.MaxYear;