Stored procedure for finding out if student is eligible for course with pre reqs - sql

Question

I have this stored procedure for a course registration system I am working on. My intention is to return a value of -1 if the query returns a course which a student has not taken according to the course pre requisites.
The pre req table only has two columns; CourseID being the course and PreReqCourse_ID being the required course for that specified course. If the student has taken all pre req courses then it should return a value of 1. I keep getting a value of -1 even when I run the query for a student who has taken the required pre req courses. Any help would be much appreciated!
CREATE PROCEDURE CheckPreReq
#StudentID INT,
#CourseID INT
AS
DECLARE #theCount INT
IF EXISTS (SELECT *
FROM PreReq
INNER JOIN Student_History ON (PreReq.Course_ID = #CourseID)
WHERE Student_History.Course_ID != PreReq.PreReqCourse_ID
AND Student_History.Student_ID = #StudentID)
BEGIN
SET #theCount =-1
END
ELSE
BEGIN
SET #theCount = 1
END
RETURN #theCount

Answer 1

Would something like this work?
DECLARE #PreReqsTotal tinyint
DECLARE #PreReqsFulfilled tinyint
-- Count pre-req courses.
SELECT #PreReqsTotal = COUNT(*)
FROM PreReq
WHERE [CourseID] = #CourseId
-- Count how many fulfilled by student.
SELECT #PreReqsFulfilled = count(*)
FROM Student_History hist
JOIN PreReq pre
on hist.Course_ID = pre.PreReqCourse_ID
WHERE pre.CourseID = #CourseID
and hist.Student_ID = #StudentID
RETURN CASE WHEN #PreReqsTotal = #PreReqsFulfilled THEN 1 ELSE -1 END
...or something like this:
IF EXISTS
(
SELECT blah.*
FROM
(
SELECT pre.*
,[PreFulfilled] = CASE WHEN hist.Course_ID is null THEN 0 ELSE 1 END
FROM PreReq pre
LEFT JOIN
Student_History hist
on pre.PreReqCourse_ID = hist.Course_ID
and hist.Student_ID = #StudentID
WHERE pre.CourseID = #CourseID
) blah
WHERE blah.[PreFulfilled] = 0 -- Unfulfilled PreReq.
)
BEGIN
RETURN -1 -- Has an unfulfilled PreReq.
END
RETURN 1 -- No unfulfilled PreReqs.

Answer 2

You should JOIN the PreReq table with the Student_History on the the PreReq.PreReqCourse_ID and Student_History.CourseID columns (take a look at my example). Then your SP should work.
--create tmp example tables
IF OBJECT_ID('tempdb..#PreReq') IS NOT NULL DROP TABLE #PreReq
CREATE TABLE #PreReq(
CourseID int,
PreReqCourse_ID int
)
--insert Course 3 which depends on Course 2 and 1 in #PreReq
INSERT INTO #PreReq
values(3,2),(3,1)
IF OBJECT_ID('tempdb..#Student_History') IS NOT NULL DROP TABLE #Student_History
CREATE TABLE #Student_History(
CourseID int not null,
StudentID int not null
);
--insert Student 1 who has visited Course 1 and 2
insert into #Student_History
VALUES(1,1),(2,1)
--declare variables
DECLARE #CourseID AS INT = 3
,#StudentID AS INT = 1
--COUNT on how many Courses #CourseID depends
,#necessaryCourses AS INT
--COUNT on how many Courses the Student has taken
,#countTakenCourses AS INT
,#theCount AS INT
SET #necessaryCourses = (SELECT count(*) FROM #PreReq WHERE CourseID = #CourseID);
SET #countTakenCourses = (
SELECT count(*)
FROM #PreReq p
--JOIN with Student_History to check if the student has visited the necessary course
JOIN #Student_History h on p.PreReqCourse_ID = h.CourseID
WHERE p.CourseID = #CourseID AND h.StudentID = #StudentID
)
IF #necessaryCourses = #countTakenCourses
BEGIN
set #theCount = 1
END
ELSE
BEGIN
set #theCount = -1
END
SELECT #theCount AS theCount