Question

There’s one table named StudentScore which has fields of: Score, CourseID, StudentID and Semester. The later three ones are the primary keys.

I want to write a stored procedure to get the average score of each student. But the rule is quite complex and I don’t know how to express it in one query. Nested query should be avoided if is possible.

Here is the rule:

If one student take a course for more than once, only the last score should be calculated.

For example, there’re following data:

StudentID    | CourseID  | Semester  | Score
1              1           1           80
1              2           1           40
1              3           1           60
1              2           2           50
1              3           2           20
2              1           1           90

The stored procedure should return:

StudentID    | AvgScore
1              50 // which is avg(80, 50, 20)
2              90

Please suggest stored procedure as efficient as possible. Thanks!

Answer 1

;WITH x AS 
(
  SELECT StudentID, Score, rn = ROW_NUMBER() OVER 
   (PARTITION BY StudentID, CourseID 
    ORDER BY Semester DESC) 
  FROM dbo.StudentScore
)
SELECT StudentID, AvgScore = AVG(Score)
FROM x
WHERE rn = 1
GROUP BY StudentID;

If you want something rounded to certain decimal places, maybe:

;WITH x AS 
(
  SELECT StudentID, Score = 1.0*Score, rn = ROW_NUMBER() OVER 
   (PARTITION BY StudentID, CourseID 
    ORDER BY Semester DESC) 
  FROM dbo.StudentScore
)
SELECT StudentID, AvgScore = CONVERT(DECIMAL(10,2), AVG(Score))
FROM x
WHERE rn = 1
GROUP BY StudentID;