Проверка наличия данных

string sql = @"INSERT INTO egais(id_document,id_sender,full_name_sender,Short_name_sender,sender_inn,sender_kpp,reg_sender,adress_sender,id_recipient,full_name_recipient,short_name_recipient,inn_recipient," +
                            "reg_recipient,adress_recipient,type_product,full_name_product,short_name_product,Alc_code,capacity,Alc_volume,id_manufacturer,full_name_manufacturer,short_name_manufacturer,inn_manufacturer, kpp_manufacturer," +
                            "reg_manufacturer,adress_manufacturer,cod_product,quantity, price_for_one) VALUES('" + a1 + "','" + a2 + "','" + a3 + "','" + a4 + "','" + a5 + "','" + a6 + "','" + a7 + "','" + a8 + "','" + a9 + "','" + a10 + "','" + a11 + "','" + a12 + "','" + a13 + "','" + a14 + "','" + a15 + "'," +
                            "'" + products[z] + "','" + a17 + "','" + a18 + "','" + a19 + "','" + a20 + "','" + a21 + "','" + a22 + "','" + a23 + "','" + a24 + "','" + a25 + "'," +
                            "'" + a26 + "','" + a27 + "','" + a28 + "','" + a29 + "','" + a30 + "')";
                            MySqlCommand cmd = new MySqlCommand(sql, mysql);
                            cmd.ExecuteNonQuery()

MySqlCommand conv = new MySqlCommand("SELECT id_document FROM egais WHERE id_document='"+a2+"'", mysql);