Course note — Slowly changing dimensions, and the "as of when" problem

Revised Slowly changing dimensions, and the "as of when" problem this week. The change was small and the reason was not.

A customer moved from Berlin to Madrid last March. Was last February's revenue German or Spanish? The answer is a design decision, and pretending otherwise is how reports stop reconciling.

The first draft explained the mechanism and stopped. What was missing was the failure: the thing that happens when you get it wrong, described concretely enough that someone recognises it later. A lesson that only teaches the correct version leaves you unable to spot the incorrect one, which is the situation you will actually be in.

It sits in the Modelling the warehouse module. The exercise is a thinking one rather than a query one — some of this material is about deciding what to compute, and that does not need a keyboard.

Ordering matters here more than in most courses. It follows Facts, dimensions, and picking a grain and leads into Why your query costs what it costs, and reading it out of sequence mostly works but costs you the setup.

Free means free, and it also means we can rewrite it whenever it is wrong. No edition, no errata PDF, nothing to repurchase. The whole course is 36 lessons and the fixes land the day we find them.