Reader question — The patterns that keep coming up

Reader question on The patterns that keep coming up, and the answer belongs somewhere more findable than an email.

A short catalogue of the query shapes that appear in real work over and over — cohorts, funnels, running totals, deduplication, pivots — with the version that actually works.

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 SQL that survives production module, and the exercise runs against saas-subscriptions. That pairing is deliberate: the dataset was built with the trap the lesson describes already in it, so the exercise fails in the instructive way rather than the confusing one.

Ordering matters here more than in most courses. It follows Writing SQL people can read, and reading the plan and leads into Testing data like you test code, 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.