Double Counting in Aggregation
Join an order to its shipments and the order amount appears once per shipment. The join is correct; the sum is not. This is where inflated totals come from.
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Fact Table Grain and Aggregation
Wrong totals rarely raise an error. See why grain is a contract, which facts you may not sum, and the join that inflates a number without warning.
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Data Grain
Learn what one row represents and why joining different grains can multiply measures.