We have an architecture where we provide each customer Business Intelligence-like services for their website (internet merchant). Now, I need to analyze those data internally (f
You will want to google Star Schema. The basic idea is to model a special data warehouse / OLAP instance of your existing OLTP system in a way that is optimized to provided the type of aggregations you describe. This instance will be comprised of facts and dimensions.
In the example below, sales 'facts' are modeled to provide analytics based on customer, store, product, time and other 'dimensions'.
You will find Microsoft's Adventure Works sample databases instructive, in that they provide both the OLTP and OLAP schemas along with representative data.