CREATE TRIGGER multipurposeCourseEnrollment
    ON courseEnrollment AFTER INSERT, DELETE, UPDATE AS
        BEGIN
            IF EXISTS(SELECT courseID FROM DELETED) AND EXISTS(SELECT courseID FROM INSERTED)
                THEN
                    DECLARE @oldCourseID INT
                    DECLARE @newCourseID INT
                    SELECT @oldCourseID = (SELECT courseID FROM DELETED)
                    SELECT @newCourseID = (SELECT courseID FROM INSERTED)
                    BEGIN
                        UPDATE courses SET courseOpenSeats = (courseOpenSeats +1)
                            WHERE courseID = @oldCourseID
                        UPDATE courses SET courseOpenSeats = (courseOpenSeats -1)
                            WHERE courseID = @newCourseID AND courseOpenSeats>0
                    END
            ELSE
            IF EXISTS(SELECT courseID FROM DELETED)
                THEN
                    DECLARE @courseIDDel INT
                    SELECT @courseIDDel = (SELECT courseID FROM DELETED)
                    BEGIN
                        UPDATE courses SET courseOpenSeats = (courseOpenSeats +1)
                            WHERE courseID = @courseIDDel
                    END
            ELSE
            IF EXISTS(SELECT courseID FROM UPDATED)
                THEN
                    DECLARE @courseID INT
                    SELECT @courseID = (SELECT courseID FROM UPDATED)
                    BEGIN


        UPDATE courses SET courseOpenSeats = (courseOpenSeats - 1)
                        WHERE courseID = @courseID AND courseOpenSeats>0
                END
        ELSE
        END)
    END;