Field note · January 27, 2025
Reader question — 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.
Reader question on Joins that do not fan out, and the answer belongs somewhere more findable than an email.
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 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 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.