Customize in purch table form

In purch table form

1-when insert new serial I need to put restriction in serial if it's already exist user can't insert it

2- I need to add another data source to

This form and use fields from this table to insert details

For example:

I will add table named details in this form fields(main category - sub category -market)

In this form have

seial number ,item number ......

And will add (main category-sub category-market)

How come user enter data and auto save in my details table by insert new row in it