private void ManagerSanPham_Load(object sender, EventArgs e)
 {
     dgvSach.DataSource         = db.SachTable();
     dgvSach.Columns[2].Visible = false;
     dgvSach.Columns[7].Visible = false;
     dgvSach.Columns[8].Visible = false;
     dgvSach.Columns[9].Visible = false;
 }