Using case when mysql - mysql

Question

I would like to query a total marks column using case and get the marks respective grades and store it as a separate column. I have been trying for a while now, but it keeps giving me errors.
SELECT gradecalc.StudentId,gradecalc.CourseCode, Total,
CASE TOTAL
WHEN >90 THEN 'AA'
WHEN <90 AND >85 THEN 'BA'
WHEN <85 AND >=80 THEN 'BB'
WHEN <80 AND >=75 THEN 'CB'
WHEN <75 AND >=70 THEN 'CC'
WHEN <70 AND >=65 THEN 'DC'
WHEN <65 AND >=60 THEN 'DD'
ELSE 'FF'
END AS Grade
FROM GRADECALC GROUP BY gradecalc.StudentId,gradecalc.CourseCode

Answer 1

The case <column> when <value> syntax can only handle equality checks. For other operators, your'd have to use a slightly different syntax:
SELECT gradecalc.StudentId,gradecalc.CourseCode, Total,
CASE
WHEN total > 90 THEN 'AA'
WHEN total < 90 AND total >= 85 THEN 'BA'
WHEN total < 85 AND total >= 80 THEN 'BB'
WHEN total < 80 AND total >=75 THEN 'CB'
WHEN total < 75 AND total >=70 THEN 'CC'
WHEN total < 70 AND total >=65 THEN 'DC'
WHEN total < 65 AND total >=60 THEN 'DD'
ELSE 'FF'
END AS Grade
FROM GRADECALC GROUP BY gradecalc.StudentId,gradecalc.CourseCode

Answer 2

You need to put the full expression in the case when. Your syntax of case <variable> when <constant> only works for constants.
Now, you can also simplify the expressions, because case is processed in order. So this is equivalent logic:
SELECT gc.StudentId, gc.CourseCode, gc.Total,
(CASE WHEN TOTAL > 90 THEN 'AA'
WHEN TOTAL > 85 THEN 'BA'
WHEN TOTAL >= 80 THEN 'BB'
WHEN TOTAL >= 75 THEN 'CB'
WHEN TOTAL >= 70 THEN 'CC'
WHEN TOTAL >= 65 THEN 'DC'
WHEN TOTAL >= 60 THEN 'DD'
ELSE 'FF'
END) AS Grade
FROM GRADECALC gc
GROUP BY gc.StudentId, gc.CourseCode ;
I don't think the GROUP BY is necessary, unless you have multiple rows for a given student and course. If you do, then you probably want AVG(Total) or something like that in the case statement.
With this formulation, you can readily see that "90" and "85" are suspicious. In fact, your original logic would give students with these marks an "FF", which I'm guessing is not the intention.
So:
SELECT gc.StudentId, gc.CourseCode, gc.Total,
(CASE WHEN TOTAL >= 90 THEN 'AA'
WHEN TOTAL >= 85 THEN 'BA'
WHEN TOTAL >= 80 THEN 'BB'
WHEN TOTAL >= 75 THEN 'CB'
WHEN TOTAL >= 70 THEN 'CC'
WHEN TOTAL >= 65 THEN 'DC'
WHEN TOTAL >= 60 THEN 'DD'
ELSE 'FF'
END) AS Grade
FROM GRADECALC gc;

Answer 3

You can try this, mate:
SELECT
gc.StudentId,
gc.CourseCode,
gc.Total,
CASE
WHEN gc.Total > 90 THEN 'AA'
WHEN gc.Total BETWEEN 85 AND 90 THEN 'BA'
WHEN gc.Total BETWEEN 80 AND 85 THEN 'BB'
WHEN gc.Total BETWEEN 75 AND 80 THEN 'CB'
WHEN gc.Total BETWEEN 70 AND 75 THEN 'CC'
WHEN gc.Total BETWEEN 65 AND 70 THEN 'DC'
WHEN gc.Total BETWEEN 60 AND 65 THEN 'DD'
ELSE 'FF'
END AS Grade
FROM
gradecalc gc
GROUP BY
gc.StudentId, gc.CourseCode;