code works to a point - postgresql

Question

The following code looks good to me but works to a point. The function should display the grade levels of students based on exam performance but it does not run the last two else statements and so, if a student scores lower than 50 the function still displays "pass".
CREATE OR REPLACE FUNCTION stud_Result(integer,numeric) RETURNS text
AS
$$
DECLARE
stuNum ALIAS FOR $1;
grade ALIAS FOR $2;
result TEXT;
BEGIN
IF grade >= 70.0 THEN SELECT 'distinction' INTO result FROM student,entry
WHERE student.sno = entry.sno AND student.sno = stuNum;
ELSIF grade >=50.0 OR grade <=70.0 THEN SELECT 'pass' INTO result FROM student,entry
WHERE student.sno = entry.sno AND student.sno = stuNum;
ELSIF grade >0 OR grade< 50.0 THEN SELECT 'fail' INTO result FROM student,entry
WHERE student.sno = entry.sno AND student.sno = stuNum;
ELSE SELECT 'NOT TAKEN' INTO result FROM student,entry
WHERE student.sno = entry.sno AND student.sno = stuNum;
END IF;
RETURN result;
END;$$
LANGUAGE PLPGSQL;
Can anyone point me to the problem?

Answer 1

This is a PostgreSQL gotcha that has tripped me up as well. You need to replace your ELSE IFs with ELSIF.
You're seeing that error because each successive ELSE IF is being interpreted as starting a nested IF block, which expects its own END IF;.
See the documentation on conditionals for more information on the proper syntax.

Answer 2

Your logic in the conditionals is a bit strange. You have these:
grade >= 70.0
grade >= 50.0 OR grade <= 70.0
grade > 0 OR grade < 50.0
Note that zero satisfies the second condition as do a lot of other values that you don't want in that branch of the conditonal. I think you want these:
grade >= 70.0
grade >= 50.0 AND grade <= 70.0
grade > 0 AND grade < 50.0
You also seem to be using your SELECTs to check if the person is in the course but if the grade is given and they're not in the course, you will end up with a NULL result. Either the "in the course" check should be outside your function or you should convert a NULL result to 'NOT TAKEN' before returning.
This looks like homework so I'm not going to be anymore explicit than this.

Answer 3

Generally, I don't think it is a good idea to hide data in code. Data belongs in tables:
SET search_path='tmp';
-- create some data
DROP TABLE tmp.student CASCADE;
CREATE TABLE tmp.student
( sno INTEGER NOT NULL
, grade INTEGER
, sname varchar
);
INSERT INTO tmp.student(sno) SELECT generate_series(1,10);
UPDATE tmp.student SET grade = sno*sno;
DROP TABLE tmp.entry CASCADE;
CREATE TABLE tmp.entry
( sno INTEGER NOT NULL
, sdate TIMESTAMP
);
INSERT INTO tmp.entry(sno) SELECT generate_series(1,10);
-- table with interval lookup
DROP TABLE tmp.lookup CASCADE;
CREATE TABLE tmp.lookup
( llimit NUMERIC NOT NULL
, hlimit NUMERIC
, result varchar
);
INSERT INTO lookup (llimit,hlimit,result) VALUES(70, NULL, 'Excellent'), (50, 70, 'Passed'), (30, 50, 'Failed')
;
CREATE OR REPLACE FUNCTION stud_result(integer,numeric) RETURNS text
AS $BODY$
DECLARE
stunum ALIAS FOR $1;
grade ALIAS FOR $2;
result TEXT;
BEGIN
SELECT COALESCE(lut.result, 'NOT TAKEN') INTO result
FROM student st, entry en
LEFT JOIN lookup lut ON (grade >= lut.llimit
AND (grade < lut.hlimit OR lut.hlimit IS NULL) )
WHERE st.sno = en.sno
AND st.sno = stunum
;
RETURN result;
END; $BODY$ LANGUAGE PLPGSQL;
-- query joining students with their function values
SELECT st.*
, stud_result (st.sno, st.grade)
FROM student st
;
But wait: you can just as well do without the ugly function:
-- Plain query
SELECT
st.sno, st.sname, st.grade
, COALESCE(lut.result, 'NOT TAKEN') AS result
FROM student st
LEFT JOIN lookup lut ON ( 1=1
AND lut.llimit <= st.grade
AND ( lut.hlimit > st.grade OR lut.hlimit IS NULL)
)
JOIN entry en ON st.sno = en.sno
;
Results:
sno | grade | sname | stud_result
-----+-------+-------+-------------
1 | 1 | | NOT TAKEN
2 | 4 | | NOT TAKEN
3 | 9 | | NOT TAKEN
4 | 16 | | NOT TAKEN
5 | 25 | | NOT TAKEN
6 | 36 | | Failed
7 | 49 | | Failed
8 | 64 | | Passed
9 | 81 | | Excellent
10 | 100 | | Excellent
(10 rows)
sno | sname | grade | result
-----+-------+-------+-----------
1 | | 1 | NOT TAKEN
2 | | 4 | NOT TAKEN
3 | | 9 | NOT TAKEN
4 | | 16 | NOT TAKEN
5 | | 25 | NOT TAKEN
6 | | 36 | Failed
7 | | 49 | Failed
8 | | 64 | Passed
9 | | 81 | Excellent
10 | | 100 | Excellent
(10 rows)

Answer 4

Get rid of the function altogether and use a query:
SELECT
s.*,
CASE e.grade
WHEN >= 0 AND < 50 THEN 'failed'
WHEN >= 50 AND < 70 THEN 'passed'
WHEN >= 70 AND <= 100 THEN 'excellent'
ELSE 'not taken'
END
FROM
student s,
entry e
WHERE
s.sno = e.sno;