public void LoadSampleData()
        {
            //create and populate the tables: from the database or using linqtosql
            List<ARecord> table1 = new List<ARecord>(); //no from 1 to  20
            for (int i = 1; i <= 20; i++)
                table1.Add(new ARecord(i, i * 5));
            List<ARecord> table2 = new List<ARecord>(); //even numbers;
            for (int i = 1; i <= 20; i++)
                if (i % 2 == 0)
                    table2.Add(new ARecord(i, i * 2));
            List<ARecord> table3 = new List<ARecord>(); //multiples of 3
            for (int i = 1; i <= 20; i++)
                if (i % 3 == 0)
                    table3.Add(new ARecord(i, i * 3));
 
            //DateTime dt = DateTime.Now;
            //TimeSpan elapsed1, elapsed2;
            //int total=10;
            //for (int i = 0; i < total; i++)
            //{
                listBox1.DataSource = CalcWithFirst(table1, table2, table3).ToList();
            //}
            //elapsed1 = DateTime.Now - dt;
            //dt = DateTime.Now;
            //for (int i = 0; i < total; i++)
            //{
            //    listBox1.DataSource = CalcWithFirst(table1, table2, table3).ToList();
            //}
            //elapsed2 = DateTime.Now - dt;
            //MessageBox.Show(string.Format("{0}\n{1}", elapsed1.TotalSeconds, elapsed2.TotalSeconds));
            
        }
        IEnumerable<ARecord> CalcWithFirst(List<ARecord> table1, List<ARecord> table2, List<ARecord> table3)
        {
            var result = from r in table1
                         let x = (from r2 in table2 select r2.id) //select t2 ids
                         let y = (from r3 in table3 select r3.id) //select t3 ids
                         where !x.Contains(r.id) && !y.Contains(r.id) //not in 2 and 3
                         select r;
            return result;
        }
        IEnumerable<ARecord> CalcWithSecond(List<ARecord> table1, List<ARecord> table2, List<ARecord> table3)
        {
            var result = from r in table1
                         let x = (from r2 in table2 where r2.id == r.id select r2.id).Count()
                         let y = (from r3 in table3 where r3.id == r.id select r3.id).Count() 
                         where x == 0 && y == 0
                         select r;
            return result;
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            LoadSampleData();
        }