Cutting a lesson down — Slowly changing dimensions, and the "as of when" problem

A note from writing Slowly changing dimensions, and the "as of when" problem, which took three passes to get to something short.

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 hard part of writing this was cutting it. The first version covered every case; the useful version covers the case you hit on a Tuesday and names the rest in a sentence. Completeness is a property of reference material, not of teaching material, and confusing the two produces something nobody finishes.

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.