public bool KiemTraTonTai(string ma)
        {
            string query = " SELECT * FROM dbo.ChungLoai  WHERE MaLoai = @ma";

            if (DatabaseDAL.readDataToTable(query, ma).Rows.Count == 0)
            {
                return(false);
            }
            return(true);
        }
        public bool KtTonTaiHangHoa(string maHang)
        {
            string query = "SELECT * FROM dbo.HangHoa WHERE MaHang=@ma";

            if (DatabaseDAL.readDataToTable(query, maHang).Rows.Count == 0)
            {
                return(false);
            }
            return(true);
        }
        public DataTable LayToanBoNhaSX()
        {
            string query = "SELECT * FROM dbo.NhaSX";

            return(DatabaseDAL.readDataToTable(query));
        }
        public DataTable LayToanBoChungLoai()
        {
            string query = "SELECT * FROM dbo.ChungLoai";

            return(DatabaseDAL.readDataToTable(query));
        }
        public DataTable timKiemNhanVien(string maNV)
        {
            string query = "SELECT * FROM dbo.NhanVien WHERE MaNV=@manv";

            return(DatabaseDAL.readDataToTable(query, maNV));
        }
        public DataTable hienThiTatCaNhanVien()
        {
            string query = "SELECT *FROM dbo.NhanVien";

            return(DatabaseDAL.readDataToTable(query));
        }