Question

I have a simple general weighed average computation in my SELECT:

SELECT ROUND(SUM((G.UnitsAcademic*GD.Grade))/SUM(G.UnitsAcademic),3) AS 'GWA'
FROM Gradesheet G 
INNER JOIN GradeSheetDetail AS GD 
    ON GD.GradesheetId=G.GradesheetId

However, some students have non-numeric grades (such as “No Exam”) or blank grades. The code above still computes and returns a value.

I would like it to return a blank (” “) when a non-numeric value gets involved in the computation. Is there a way for SQL to do this?

Answer 1

Try this:

SELECT 
    CASE 
        WHEN 
            MAX(CASE WHEN GD.Grade = 'No Grade' OR GD.Grade = ' ' THEN 1 ELSE 0 END) = 1 THEN ' '
        ELSE 
            ROUND(SUM((G.UnitsAcademic*GD.Grade))/SUM(G.UnitsAcademic),3)  
    END AS 'GWA'  
FROM Gradesheet G  
INNER JOIN GradeSheetDetail AS GD  
      ON GD.GradesheetId = G.GradesheetId 

First, you need to check if there is at least one non-numeric field, if ‘yes’ then return blank, else calculate aggregate.