public static void InsertStudentperCourse()
        {
            Select.selectCourses();
            Console.WriteLine("Choose one course id: ");
            int courseid = Convert.ToInt32(Console.ReadLine());

            Select.selectStudents();
            Console.WriteLine("Choose one student id: ");
            int studentid = Convert.ToInt32(Console.ReadLine());

            //INSERT STUDENT_COURSE
            string connectionString = @"Data Source=DESKTOP-JBG6OHV\SQLEXPRESS;Initial Catalog=SCHOOL;Integrated Security=True";
            string sqlqr            = "INSERT INTO COURSE_STUDENT(CID,SID) VALUES(@cid,@sid)";
            string sqlqrtest        = "SELECT COUNT(*) FROM COURSE_STUDENT WHERE CID=@idc AND SID=@ids";

            try
            {
                using (SqlConnection cn = new SqlConnection(connectionString))
                {
                    cn.Open();

                    SqlCommand cmfinder = new SqlCommand(sqlqrtest, cn);
                    cmfinder.Parameters.Add(new SqlParameter("idc", courseid));
                    cmfinder.Parameters.Add(new SqlParameter("ids", studentid));

                    int count = (int)cmfinder.ExecuteScalar();
                    if (count >= 1)
                    {
                        throw new Exception("This id already exists.");
                    }
                    SqlCommand cm = new SqlCommand(sqlqr, cn);
                    cm.Parameters.Add(new SqlParameter("cid", courseid));
                    cm.Parameters.Add(new SqlParameter("sid", studentid));

                    cm.ExecuteNonQuery();
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
        }
        public static void InsertTrainer()
        {
            //INSERT TRAINER & INSERT TRAINERS PER COURSE
            string connectionString = @"Data Source=DESKTOP-JBG6OHV\SQLEXPRESS;Initial Catalog=SCHOOL;Integrated Security=True";
            string sqlqr            = "INSERT INTO TRAINER (IDTRAINER,FIRSTNAME,LASTNAME,SUBJECT,COURSEID) VALUES(@id,@fname,@lname,@subject,@idc)";
            string sqlqr1           = "INSERT INTO TRAINER (IDTRAINER,FIRSTNAME,LASTNAME,SUBJECT) VALUES(@id1,@fname1,@lname1,@subject1)";
            string sqlqrtest        = "SELECT COUNT(*) FROM TRAINER WHERE IDTRAINER= @id2";

            try
            {
                using (SqlConnection cn = new SqlConnection(connectionString))
                {
                    cn.Open();
                    Console.WriteLine("If you want to pair a trainer with a course press 1 else press 2:");
                    int option = Convert.ToInt32(Console.ReadLine());

                    if (option == 1)
                    {
                        Console.WriteLine("Enter Trainer ID: ");
                        int        id       = Convert.ToInt32(Console.ReadLine());
                        SqlCommand cmfinder = new SqlCommand(sqlqrtest, cn);
                        cmfinder.Parameters.Add(new SqlParameter("id2", id));

                        int count = (int)cmfinder.ExecuteScalar();
                        if (count >= 1)
                        {
                            throw new Exception("This id already exists.");
                        }

                        Console.WriteLine("Enter firstname: ");
                        string fname = Console.ReadLine();

                        Console.WriteLine("Enter lastname: ");
                        string lname = Console.ReadLine();

                        Console.WriteLine("Enter subject: ");
                        string subject = Console.ReadLine();

                        Select.selectCourses();
                        Console.WriteLine();
                        Console.WriteLine();

                        Console.WriteLine("Enter Course id: ");
                        int idC = Convert.ToInt32(Console.ReadLine());

                        SqlCommand cm = new SqlCommand(sqlqr, cn);
                        cm.Parameters.Add(new SqlParameter("@id", id));
                        cm.Parameters.Add(new SqlParameter("fname", fname));
                        cm.Parameters.Add(new SqlParameter("lname", lname));
                        cm.Parameters.Add(new SqlParameter("subject", subject));
                        cm.Parameters.Add(new SqlParameter("idc", idC));

                        cm.ExecuteNonQuery();
                    }
                    else if (option == 2)
                    {
                        Console.WriteLine("Enter Trainer ID: ");
                        int        id1      = Convert.ToInt32(Console.ReadLine());
                        SqlCommand cmfinder = new SqlCommand(sqlqrtest, cn);
                        cmfinder.Parameters.Add(new SqlParameter("id2", id1));

                        int count = (int)cmfinder.ExecuteScalar();
                        if (count >= 1)
                        {
                            throw new Exception("This id already exists.");
                        }

                        Console.WriteLine("Enter firstname: ");
                        string fname1 = Console.ReadLine();

                        Console.WriteLine("Enter lastname: ");
                        string lname1 = Console.ReadLine();

                        Console.WriteLine("Enter subject: ");
                        string subject1 = Console.ReadLine();

                        SqlCommand cm = new SqlCommand(sqlqr1, cn);
                        cm.Parameters.Add(new SqlParameter("id1", id1));
                        cm.Parameters.Add(new SqlParameter("fname1", fname1));
                        cm.Parameters.Add(new SqlParameter("lname1", lname1));
                        cm.Parameters.Add(new SqlParameter("subject1", subject1));

                        cm.ExecuteNonQuery();
                    }
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
        }
        static void Main(string[] args)
        {
            int option = 0;

            while (option != 3)
            {
                Console.WriteLine();
                Console.WriteLine("-------SCHOOL MENU-------");
                Console.WriteLine("Choose 1-3 for: ");
                Console.WriteLine("1.View data");
                Console.WriteLine("2.Insert data");
                Console.WriteLine("3.Exit");
                Console.WriteLine();
                option = Convert.ToInt32(Console.ReadLine());
                switch (option)
                {
                case 1:
                    option = 0;
                    Console.WriteLine();
                    Console.WriteLine("------------view data-------------");
                    Console.WriteLine("Choose 1-9 for: ");
                    Console.WriteLine("1.Student");
                    Console.WriteLine("2.Trainer");
                    Console.WriteLine("3.Assigment");
                    Console.WriteLine("4.Course");
                    Console.WriteLine("5.Students per course");
                    Console.WriteLine("6.Trainers per course");
                    Console.WriteLine("7.Assigments per course");
                    Console.WriteLine("8.Assigments per student");
                    Console.WriteLine("9.Students that belongs to more than one course");
                    Console.WriteLine();
                    option = Convert.ToInt32(Console.ReadLine());
                    switch (option)
                    {
                    case (1):
                        Select.selectStudents();
                        break;

                    case (2):
                        Select.selectTrainers();
                        break;

                    case (3):
                        option = 0;
                        Select.selectAssignments();
                        break;

                    case (4):
                        Select.selectCourses();
                        break;

                    case (5):
                        Select.selectStudentsperCourse();
                        break;

                    case (6):
                        Select.selectTrainersperCourse();
                        break;

                    case (7):
                        Select.selectAssignmentsperCourse();
                        break;

                    case (8):
                        Select.selectAssignmentsperCourseperStudent();
                        break;

                    case (9):
                        Select.selectStudentsBelongsToMoreThanOneCourse();
                        break;

                    default:
                        Console.WriteLine("Something went wrong");
                        break;
                    }
                    break;

                case 2:
                    option = 0;
                    Console.WriteLine();
                    Console.WriteLine("------------insert data-------------");
                    Console.WriteLine("Choose 1-7 for: ");
                    Console.WriteLine("1.Student");
                    Console.WriteLine("2.Trainer");
                    Console.WriteLine("3.Assigment");
                    Console.WriteLine("4.Course");
                    Console.WriteLine("5.Students per course");
                    Console.WriteLine("6.Trainers per course");
                    Console.WriteLine("7.Assigments per student");
                    Console.WriteLine();
                    option = Convert.ToInt32(Console.ReadLine());
                    switch (option)
                    {
                    case (1):
                        Insert.InsertStudent();
                        break;

                    case (2):
                        Insert.InsertTrainer();
                        break;

                    case (3):
                        option = 0;
                        Insert.InsertAssignment();
                        break;

                    case (4):
                        Insert.InsertCourse();
                        break;

                    case (5):
                        Insert.InsertStudentperCourse();
                        break;

                    case (6):
                        Insert.InsertTrainer();
                        break;

                    case (7):
                        Insert.InsertAssignmentperStudentperCourse();
                        break;

                    default:
                        Console.WriteLine("Something went wrong");
                        break;
                    }
                    break;
                }
            }
        }