>Hi,
>I know primary / candicate key can prevent duplication, but the error only occured after save(). I want to prompt for uniqueness error immediately after user key in data in BizObjAddForm.
>
>Any ideas?

Hi, John.
I would

 select mytable again alias mytable_2 in 0
* do the duplication check
use in mytable_2
* inform the user 

All the best.