URGENT! Infopath formula or filter or new rule needed

I have been charged with creating an online form that will be accessible on sharepoint. The form is basically an asset inventory form. Large numbers of techs will be manually verifying existing old database information manually by completing this new form online.

I have a copy of the old database in the form of an Excel dump. Ive uploaded the Excel file as a separate list in sharepoint.

I want the following:
I have a set of fields (asset tag, serial #, model#, etc.). I want to say that if user enters an asset # that already exists in the sharepoint list, then prepopulate my form's serial#, model# fields, with the values we already know.

Simple enough. I created a Rule on my asset# field in my infopath form.
It says if myasset# = datasourceasset#, then fire more rules: a) set mymodel to datasource model, b) set myserial# to datasource#, etc.

When I run the form it does prepopulate information in the addl fields, but it picks the first row of data from the existing sharepoint list, not the data to the specific asset# that was entered by the user.

What I really want to say is:
myserial# field should equal datasourceserial# where myassetID equals datasourceassetID.

I know I can create a Filter on a Value in a Rule, but Im not sure how to build the right filter. In my mind, I would like a Where clause, as in SQL, but I dont know the corresponding function using InfoPath.

Any help appreciated. Need to figure this out today...