how do i make a grading system using cases? - postgresql

Question

pretty sure this is a pretty simple issue:
I need to create an exam results table for a student, but it needs to meet the conditions of If the student is awarded a grade of 70% or more then the result is to be shown as 'Distinction', a grade of at least 50% but less than 70% is to be shown as 'Pass' and grades below 50% are to be shown as 'Fail'. If the student has not taken the examination then the result is shown as 'Not taken'.
I'm having issues with my code which i can't seem to get right:
CREATE VIEW results_table
AS SELECT entry.excode, sname, student.sno, entry.egrade, result AS entry
FROM student
JOIN entry
ON student.sno = entry.sno
GROUP BY entry.excode, sname, student.sno, entry.egrade, result
SELECT result
CASE result
WHEN entry.egrade >= 70 THEN 'Distinction'
WHEN entry.egrade >= 50 THEN 'Pass'
WHEN entry.egrade < 50 THEN 'Fail'
WHEN entry.egrade = NULL THEN 'Not Taken'
END;
ORDER BY entry.excode,sname
could somebody please tell me where i'm going wrong?

Answer 1

There are two types of SQL CASE statement, as documented for PostgreSQL here:
CASE WHEN condition THEN result ELSE default END
CASE expression WHEN value THEN result ELSE default END
The first form tests a series of (unrelated) conditions, and returns the result for the first one which is true, or the ELSE if none are. The second form is a short-hand for testing multiple conditions against a single value; CASE x WHEN y THEN 'y' WHEN z THEN 'z' END is short-hand for CASE WHEN x = y THEN 'y' WHEN x = z THEN 'z' END.
In your code, you're muddling the two syntaxes; you can't use the short-hand because you're doing something more complex than = in the conditions, so you need to leave out the word result:
CASE
WHEN entry.egrade >= 70 THEN 'Distinction'
WHEN entry.egrade >= 50 THEN 'Pass'
WHEN entry.egrade < 50 THEN 'Fail'
WHEN entry.egrade IS NULL THEN 'Not Taken'
END
(Note: I've also changed = NULL to IS NULL, since no value is ever "equal to" NULL, because it means "we don't know what the value is equal to".)
You then need to put that into a SELECT query in the same place you'd select a normal column.
So starting from this simple query...
SELECT
entry.egrade as result
FROM
entry;
...you can put your CASE statement in like this:
SELECT
CASE
WHEN entry.egrade >= 70 THEN 'Distinction'
WHEN entry.egrade >= 50 THEN 'Pass'
WHEN entry.egrade < 50 THEN 'Fail'
WHEN entry.egrade IS NULL THEN 'Not Taken'
END as result
FROM
entry;