Cutting a lesson down — Testing data like you test code

A note from writing Testing data like you test code, which took three passes to get to something short.

Your transformation logic is tested by unit tests. The data flowing through it is not, and that is where the failures actually happen.

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 Data quality and trust 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 The patterns that keep coming up and leads into The anatomy of a silent failure, 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.