if cod_barra.text<>null then
begin
query1.close;
query1.params[0].value:=dm.stock.fieldvalues['cod_barra'];
query1.open;
query1.First;
While not (query1.Eof) Do
Begin
if cod_barra.text=query1.fieldvalues['cod_barra'] then
begin
showmessage('el codigo ingresado ya existe');
cod_barra.setfocus;
end;
query1.Next
end else
guardar.click;
end;