Custom Aggregation Methods for related tables with something similar Group By functionality in SQL

With the ability to relate the tables there should also be the ability to customize the aggregations.   We have several referential tables which we have a one to many relationship between them however we only want to have the aggregation affect the totals being when the referential tables are used.  This would happen say if you want to run reports for where dealers are assigned to try and sell into a zip code.   if you want to see the zip code then you only want the totals in the area for that zip code.   If you want to do a report to show the area of responsibility for a dealer then you want each dealer to have the totals for that area in their report.