update employee
set dept_code = emp_code

간단하죠??

procedure TForm1.Button1Click(Sender: TObject);
begin
  with query1 do begin
    Close;
    SQL.Clear
    SQL.Add(' Select * from employee ');
    Open;

    While not EOF do begin
      With Query2 do begin
        Close;
        SQL.Clear;
        SQL.Add(' Update employee ');
        SQL.Add(' set ');
        SQL.Add(' dept_code=:dep_code ');
        SQL.Add(' where emp_code=:empcode ');
        Prapare;

        //dept_code와 emp_code의 데이터형에 맞게 적어주세요.
        ParamByName('dept_code').AsString := Query1.FieldByName('emp_code').AsString;
        ParamByName('emp_code').AsString := Query1.FieldByName('emp_code').AsString;

        ExecSQL;
      end;
      Next;
    end;
  end;
  ShowMessage('Completed');
end;

참고해 보세요..