Automatic table update while creating the new record

Hi,

I'm working on inserting the value on the table field by depend upon one more field on the table.

For example:

If I enter the "SOP Id", the field "SOP Category" have to automatically assign as the "Sales training group" by validating the previous record of the "SOP Id". This is basically called as the revision thing.

Now I am selecting by clicking the field "SOP Category", but that have to assign it automatically as soon as I enter "SOP Id". It have to check the previous record of the "SOP Id"  and automatically the SOP Category have to assign(Which is assigned to the previous one).

Can any one please guide me, how to acheive this?