> 2. Almost at the same time (before John post his new enterprise),
> the user Peter begins to add a new enterprise. The application
> propose automatically the CODE 101 (the same as the one that is
> using John) and Peter begins to add (in a grid) the employees.
> The field ENTERPRCODE of the employees he is adding will have the
> value 101.

I'm afraid this is your design flaw. The application should not propose 101
to Peter, since it's taken by John already. Use generators and this will
never happen.
Alan