Question

I’m working on a stored procedure for an academic institution that would allow advisors to filter their student lists by a number of criteria. Recently, the advisors asked me to give them the option of filtering students by GPA. This was perfectly easy until they asked for an option to determine the inequality.

Here’s a truncated copy of the query I’m using, and the corresponding error message in SQL.

DECLARE @Inequality CHAR(2)
SET @Inequality = '>='

DECLARE @Gpa DECIMAL(3,1)
SET @Gpa = 2.0

SELECT DISTINCT
    mem.StudentID,
    SUM(GradePointValue * Credits) / SUM(Credits) AS GPA
FROM <snip>
WHERE <snip>
GROUP BY mem.StudentID
HAVING 
    CASE @Inequality
        WHEN '>=' THEN SUM(GradePointValue * Credits) / SUM(Credits) >= @Gpa
        WHEN '>'  THEN SUM(GradePointValue * Credits) / SUM(Credits) >  @Gpa
        WHEN '<'  THEN SUM(GradePointValue * Credits) / SUM(Credits) <  @Gpa
        WHEN '<=' THEN SUM(GradePointValue * Credits) / SUM(Credits) <= @Gpa
    END 
ORDER BY GPA desc

When I try to execute this query, SSMS gives me the following error message: Incorrect syntax near '>'., referencing the first line of the CASE statement in the HAVING clause, >= @GPA

I’ve tried rewriting the HAVING clause to:

HAVING SUM(GradePointValue * Credits) / SUM(Credits)
    CASE @Inequality
        WHEN '>=' THEN >=
        ...
    END @Gpa

This produces another error message: Incorrect syntax near the keyword 'CASE'.

I know I could copy and paste the entire SQL query four times and change only the inequality in the HAVING clause in each query, but that’s messy (not that this isn’t — it just doesn’t smell as bad to me) and I’d like to avoid it if I can. Is there any way for me to pull off my attempt above?

Answer 1

;WITH cte As
(
SELECT /*Don't need DISTINCT with GROUP BY*/
    mem.StudentID,
    SUM(GradePointValue * Credits) / SUM(Credits) AS GPA
FROM <snip>
WHERE <snip>
GROUP BY mem.StudentID
)
SELECT StudentID, GPA
FROM cte WHERE (GPA > @Gpa AND LEFT(@Inequality, 1) = '>')
            OR (GPA = @Gpa AND RIGHT(@Inequality,1) = '=')
            OR (GPA < @Gpa AND LEFT(@Inequality,1) = '<')