//Sửa
        public void sua(EC_tb_Chamcong key)
        {
            string sql = (@"UPDATE dbo.CHAMCONG
            SET NghiCoPhep =N'" + key.NGHICOPHEP + "' where  MaChamCong ='" + key.MACHAMCONG + "'");

            cn.ExcuteNonQuery(sql);
        }
 // Xóa
 public void xoa(EC_tb_Chamcong key)
 {
     cn.ExcuteNonQuery("DELETE FROM dbo.CHAMCONG WHERE [MaChamCong] = '" + key.MACHAMCONG + "'");
 }
 // Thêm mới
 public void themmoi(EC_tb_Chamcong key)
 {
     cn.ExcuteNonQuery(@"INSERT INTO dbo.CHAMCONG
               (MaChamCong,MaNhanVien,NgayChamCong,NghiCoPhep) VALUES   ('" + key.MACHAMCONG + "','" + key.MANHANVIEN + "','" + key.NGAYCHAMCONG + "',N'" + key.NGHICOPHEP + "')");
 }
 //Sửa
 public void sua(EC_tb_Chamcong key)
 {
     val.sua(key);
     MessageBox.Show("Đã Sửa Thành Công", "Chú Ý", MessageBoxButtons.OK, MessageBoxIcon.Information);
 }
 // Xóa
 public void xoa(EC_tb_Chamcong key)
 {
     val.xoa(key);
 }
 // Thêm mới
 public void themmoi(EC_tb_Chamcong key)
 {
     val.themmoi(key);
     MessageBox.Show("Đã Lưu Thành Công", "Chú Ý", MessageBoxButtons.OK, MessageBoxIcon.Information);
 }