See VFP HElp for :
CREATE TRIGGER ON TableName
FOR DELETE | INSERT | UPDATE AS lExpression

For instance you may use custom routine/program for lExpression.
>Hi All,
>
>I'm stuck on this very basic (at least I think it is) VFP database design issue.
>
>I've got a table with three fields, LENGTH, WIDTH and AREA. I'd like to have the AREA field automagically update its value with the product of LENGTH and WIDTH whenever either of those two values change.
>
>This update must occur under the following circumstances:
>
>1) the table is browsed to make edits to the LENGTH and/or WIDTH columns.
>2) the table is appended to.
>3) the LENGTH and/or WIDTH is modified on a form containing controls bound to them.
>
>I believe that I need to do something in the database container to "trigger" this but where and how?
>
>TIA.
>
>Mike