Field note · July 27, 2025
Cutting a lesson down — Joins that do not fan out
The most common wrong number in the world is caused by a join that multiplied rows, and the person who wrote it had no way to notice.
A note from writing Joins that do not fan out, which took three passes to get to something short.
The most common wrong number in the world is caused by a join that multiplied rows, and the person who wrote it had no way to notice.
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 SQL that survives production module, and the exercise runs against retail-orders. 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 One metric, one definition and leads into Window functions, properly, 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.