Cutting a lesson down — Writing SQL people can read, and reading the plan

A note from writing Writing SQL people can read, and reading the plan, which took three passes to get to something short.

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 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. 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.