Field note · September 8, 2025
Course note — Writing SQL people can read, and reading the plan
The query you write twice a year is written for the machine. The one in a production model is written for the person debugging it at 3am, who might be you.
Revised Writing SQL people can read, and reading the plan this week. The change was small and the reason was not.
The query you write twice a year is written for the machine. The one in a production model is written for the person debugging it at 3am, who might be you.
The first draft explained the mechanism and stopped. What was missing was the failure: the thing that happens when you get it wrong, described concretely enough that someone recognises it later. A lesson that only teaches the correct version leaves you unable to spot the incorrect one, which is the situation you will actually be in.
It sits in the SQL that survives production 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 Window functions, properly and leads into The patterns that keep coming up, 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.