how to perform a database like query

Hi Team - 

How would I perform a db like query through the formula of an object?

Specifically, I want the % completed field to be populated automatically based on the aggregate value of a subset of fields.

E.g. We have leads listed in the Customers module that are of 4 categories (A, B, C, D)

Each lead has a status (1, 2, 3, 4, 5), 1 being its a prospective client,  5 being we have closed a deal.

I want the field which say.

Select all customers with category A, and average their status.

Is there a way to loop through all projects?

If so, I can take it from there...

Thanks,

Philip