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

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

Solution

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