SQL: How do I loop through the results of a SELECT statement? - sql

Question

How do I loop through the results of a SELECT statement in SQL? My SELECT statement will return just 1 column but n results.
I have created a fictional scenario below complete with the Pseudo code of what I'm trying to do.
Scenario:
Students are registering for their classes. They submit a form with multiple course selections (ie. select 3 different courses at once). When they submit their registration I need to ensure there is still room left int the courses they have selected (note I will do a similar check before presenting them with course selection UI but I need to verify afterwards in case somebody else has gone in and swipped up the remaining spots).
Pseudo Code:
DECLARE #StudentId = 1
DECLARE #Capacity = 20
-- Classes will be the result of a Select statement which returns a list of ints
#Classes = SELECT classId FROM Student.CourseSelections
WHERE Student.CourseSelections = #StudentId
BEGIN TRANSACTION
DECLARE #ClassId int
foreach (#classId in #Classes)
{
SET #SeatsTaken = fnSeatsTaken #classId
if (#SeatsTaken > #Capacity)
{
ROLLBACK; -- I'll revert all their selections up to this point
RETURN -1;
}
else
{
-- set some flag so that this student is confirmed for the class
}
}
COMMIT
RETURN 0
My real problem is a similar "ticketing" problem. So if this approach seems very wrong please feel free to recommend something more practical.
EDIT:
Attempting to implement the solution below. At this point it doesn't work. Always returns "reserved".
DECLARE #Students TABLE
(
StudentId int
,StudentName nvarchar(max)
)
INSERT INTO #Students
(StudentId ,StudentName)
VALUES
(1, 'John Smith')
,(2, 'Jane Doe')
,(3, 'Jack Johnson')
,(4, 'Billy Preston')
-- Courses
DECLARE #Courses TABLE
(
CourseId int
,Capacity int
,CourseName nvarchar(max)
)
INSERT INTO #Courses
(CourseId, Capacity, CourseName)
VALUES
(1, 2, 'English Literature'),
(2, 10, 'Physical Education'),
(3, 2, 'Photography')
-- Linking Table
DECLARE #Courses_Students TABLE
(
Course_Student_Id int
,CourseId int
,StudentId int
)
INSERT INTO #Courses_Students
(Course_Student_Id, StudentId, CourseId)
VALUES
(1, 1, 1),
(2, 1, 3),
(3, 2, 1),
(4, 2, 2),
(5, 3, 2),
(6, 4, 1),
(7, 4, 2)
SELECT Students.StudentName, Courses.CourseName FROM #Students Students INNER JOIN
#Courses_Students Courses_Students ON Courses_Students.StudentId = Students.StudentId INNER JOIN
#Courses Courses ON Courses.CourseId = Courses_Students.CourseId
DECLARE #StudentId int = 4
-- Ideally the Capacity would be database driven
-- ie. come from the Courses.Capcity.
-- But I didn't want to complicate the HAVING statement since it doesn't seem to work already.
DECLARE #Capacity int = 1
IF EXISTS (Select *
FROM
#Courses Courses INNER JOIN
#Courses_Students Courses_Students ON Courses_Students.CourseId = Courses.CourseId
WHERE
Courses_Students.StudentId = #StudentId
GROUP BY
Courses.CourseId
HAVING
COUNT(*) > #Capacity)
BEGIN
SELECT 'full' as Status
END
ELSE BEGIN
SELECT 'reserved' as Status
END

Answer 1

No loop needed. You're looking at a standard aggregate with COUNT and GROUP.
Of course, some details are needed but the principle is this...
DECLARE #StudentId = 1
DECLARE #Capacity = 20
-- Classes will be the result of a Select statement which returns a list of ints
IF EXISTS (SELECT *
FROM
Student.CourseSelections CS
JOIN
---this is where you find out course allocations somehow
ClassTable C ON CS.classId = C.classId
WHERE
Student.CourseSelections = #StudentId
GROUP BY --change this, it depends on where you find out course allocations
ClassID
HAVING
COUNT(*) > #Capacity)
'no'
ELSE
'yes'
Edit:
I've changed the link table. Course_Student_ID is usually not needed in link tables.
The JOIN now
gets the courses for that student
then looks at all students on this course and compares to capacity
Cut down version of above:
...
-- Linking Table
DECLARE #Courses_Students TABLE (
,CourseId int
,StudentId int)
INSERT INTO #Courses_Students
(StudentId, CourseId)
VALUES (1, 1), (1, 3), (2, 1), (2, 2), (3, 2), (4, 1), (4, 2)
DECLARE #StudentId int = 4
--straight list
SELECT
C.CourseName, C.Capacity, COUNT(*)
FROM
#Courses_Students CSThis
JOIN
#Courses C ON CSThis.CourseId = C.CourseId
JOIN
#Courses_Students CSOthers ON CSOthers.CourseId = C.CourseId
WHERE
CSThis.StudentId = #StudentId
GROUP BY
C.CourseName, C.Capacity
--oversubscribed list
SELECT
C.CourseName, C.Capacity, COUNT(*)
FROM
#Courses_Students CSThis
JOIN
#Courses C ON CSThis.CourseId = C.CourseId
JOIN
#Courses_Students CSOthers ON CSOthers.CourseId = C.CourseId
WHERE
CSThis.StudentId = #StudentId
GROUP BY
C.CourseName, C.Capacity
HAVING
COUNT(*) > C.Capacity

Answer 2

Avoid looping through result sets in SQL as much as you can. If you really can't (if you really are a standard programmer but profession leads you into SQL) use cursors. They don't smell nice, but are unavoidable at times.

Answer 3

Another option would be to implement a CHECK Constraint on your table that contains the Course information. The check constraint could call your existing function to check that there are free seats.
Wrap all of your Inserts/Updates in to one transaction. If any of the Inserts/Updates fails then the entire transaction will be rolled back.