say I enter data from delphi, when I enter the sample Primary Key into one table, exception error message will raise because primary key must be unique (of course :)), but exceptions will crash the whole program, how can i transform those exceptions into a Message form that will not crash the program? like a validation. Thanks

The best way is using try.. except to handle this error. If you want to validate the input data, you can do this with BeforePost event.

If you want to validate the input data, you can do this with BeforePost event. <--- Can u explain a bit more about this one? how show the coding looks like?

Add some code into BeforePost event like :    

procedure TForm1.Table1BeforePost(DataSet: TDataSet);
var
  Qry: TQuery;
begin
  If Table1.State in [dsInsert] then
  begin
    Qry:= TQuery.Create(nil);
    try
      Qry.SQL.Add('select top 1 code from tableA where col_pk=''' Table1.FieldByName('col_pk').AsString ''' ');
      Qry.DatabaseName := Table1.DatabaseName;
      Qry.Open;
      If not Qry.IsEmpty then
      Begin
        ShowMessage('the same pk data');
        Abort;
      End;
    finally
      Qry.Free;
    end;
  end;
end;

i think i stick with using try.. except just making sure i am doing this right.. i am trying to prevent exceptation message for duplicate primary key which is a combination of (CentreNumber, CandidateNumber) so: try tExamRecord.FieldByName('CandidateNumber').AsString := Can_No.Text; tExamRecord.FieldByName('CentreNumber').AsString := Centre_No.Text; except showmessage('This record already exist!'); but it doesn't seems to work, why?

begin tStudent.Append; tStudent.FieldByName('CandidateName').AsString := Can_Name.Text; try tStudent.FieldByName('CandidateNumber').AsString := Can_no.Text; tStudent.FieldByName('CentreNumber').AsString := Centre_No.Text; except ShowMessage('The record already exist'); end; tStudent.FieldByName('CentreName').AsString := Centre_Name.Text; tStudent.Post; end; This is the full coding

I have modiftied the coding further, but it still couldn't block the exception message, when I enter the same primary key into the table begin try tStudent.Append; tStudent.FieldByName('CandidateName').AsString := Can_Name.Text; tStudent.FieldByName('CandidateNumber').AsString := Can_no.Text; tStudent.FieldByName('CentreNumber').AsString := Centre_No.Text; tStudent.FieldByName('CentreName').AsString := Centre_Name.Text; tStudent.Post; except ShowMessage('Record Already Exist!'); end; end;

the exception message is debugger message but not application message in your case. Run the program from the exe (i.e. not in Delphi) and there should be no exception. Instead, your message will be shown.