Reader question — Slowly changing dimensions, and the "as of when" problem

Reader question on Slowly changing dimensions, and the "as of when" problem, and the answer belongs somewhere more findable than an email.

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 question was, roughly, "when does this stop applying?" — which is the right question and the one lessons routinely fail to answer. Every technique has a range of validity, and stating it is what separates a lesson from a recipe.

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.