SQL: SELECT, WHERE, JOIN, GROUP BY
By the end of today you can write a query joining several tables with filtering and aggregation, and explain in what order the database conceptually evaluates the clauses.
YesterdayOn Day 41 you designed tables and relationships. Today you ask them questions.
TomorrowTomorrow you find out why some of those questions are slow, and what an index does about it.
Why this matters
SQL has outlived every ORM and framework fashion of the last thirty years. Reading it fluently means you can understand what any data layer is doing underneath.
- SELECT and WHERE
- Joins
- Grouping and aggregation
- Evaluation order
Learn it
80 minCopy this into Claude or ChatGPT. It quizzes you before it explains anything, which is deliberate. The resources under it are how you check what it told you.
Today's Master Prompt
Free · sign inA prompt written for this day alone: your level, the exact scope, what to leave out, and an instruction to quiz you before it explains anything. Paste it into Claude or ChatGPT and it teaches you today's material.
Check it against something that is not a model
An assistant can be fluent and wrong, and on a topic you met today you will not catch it. These cover the same ground and were made by people who do this for a living, so they are what you hold the explanation up against. They are other people's work and we only link to them, so judge them for yourself.
5 hand-picked resources
Free · sign inVideos, official docs and articles covering the same ground, each opened and annotated by hand. They are what you check the assistant against on a day you cannot yet catch it being wrong.
Build it
50 minUsing your library schema, write queries answering: which members currently have a book out, how many times each book has been borrowed, which books have never been borrowed, and which members have borrowed more than three books. Predict the row count of each before running it.
Recall it
20 minAnswer out loud, reveal, then mark honestly whether you had it. That score is the only thing on this page you do not get to choose.
5 recall questions
Free · sign inQuestions you answer from memory, then grade yourself against the real answer. The score is carried into the mastery rating below it, so an honest miss cannot quietly become a tick.
Rate it
Completion and mastery are tracked separately. Be honest, because an inflated rating only means the concept resurfaces sooner.
Mastery tracking
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Recap
- 01SELECT runs late, which explains most clause confusion
- 02INNER drops unmatched rows; LEFT keeps them with NULLs
- 03Grouping means each row stands for many, so columns must be grouped or aggregated
- 04COUNT(*) and COUNT(column) differ wherever NULL exists
Your progress
Free · sign inMark days complete, pick up where you left off across devices, and watch completion and mastery diverge. Free, and the account exists only so ninety days of work cannot vanish with a cleared browser.