“Select the business process. Declare the grain. Identify the dimensions. Identify the facts.”Ralph Kimball, Margy Ross · The Data Warehouse Toolkit · 2013 · Kimball Group, Dimensional Modeling Techniques — Four-Step Dimensional Design Process (aceiași pași ca în The Data Warehouse Toolkit, ed. a 3-a, cap. 3)
The grain is a sentence: "one row = one receipt line, in one store, on one day".
Kimball's four steps come in that order for a reason. The business process is something that happens — a sale, a payment, a shipment — not a department. The grain is the sentence that says what one row of the fact table stands for: "one receipt line, in one store, on one day". Only then come the dimensions — who, what, where, when, the descriptive context — and the facts, the numeric measures that can be added up at the chosen grain. The result is the star schema: a narrow, long fact table with foreign keys to wide, denormalised dimensions, each carrying a surrogate key (an integer minted by the warehouse, not the code from the source). The classic mistake is to mix grains — order lines and order totals in the same table — and get doubled sums in the first report. The second mistake is three slightly different "Customer" dimensions; conformed dimensions, shared by every process, are what make two reports comparable.
Why it matters A fact table with an undeclared grain produces figures that contradict each other across reports, and nobody can say which one is right.