Fan-out from duplicate rows before joining

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When the right side of a join has one row and the left side has N rows, the join produces N rows - one for each left row matched. This is called fan-out. 3 orders all match the single customer row, so you get 3 result rows. Fan-out becomes a bug when you accidentally join to a non-unique table - for example, joining to a prices table that has multiple effective-dated rows and forgetting to filter to the current price, which inflates your row count. Always check cardinality before joining.