SQL-FUNDAMENTALS5 MIN READ
Choose the grain before you group
State the grain of a query before writing GROUP BY.
GROUP BY is not just a syntax requirement. It is a grain declaration. The selected dimensions define the result rows, and the aggregates summarize whatever rows arrived after FROM, JOIN, and WHERE. That makes grain the control knob for correctness. Before optimizing or styling a query, say the grain in business language, then verify that the joins preserve or intentionally change it.
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