After learning the basics of dimensional modeling, the first real world challenge we face is understanding when and how to design multiple fact tables. Until we learn to think in dimensional terms, the choice can be difficult.
You probably need different fact tables if:
- You have measurements with different periodicity
- You have measurements with different levels of detail
The first guideline suggests that if facts do not describe the same event, they probably belong in different fact tables. For example, orders and shipments do not always happen at the same time. Order dollars and shipment dollars belong in separate fact tables.
The second guideline pertains to facts that do describe the same events. Information about an order and information about an order line are ostensibly available at the same time, but they have different levels of detail. If there are facts at both of these levels, there will need to be multiple fact tables.
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