From your first SELECT to window functions and the questions interviews love: 12 chapters of short lessons and 108 exercises on a real Postgres running in your browser. Every query can be X-rayed, so you see each clause work on the rows.
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1 · Read a short lesson
One idea at a time, on the café's own data, with the trap people fall into.
2 · Solve, and be told why
Wrong answers get a reason: “a join counted orders twice”, “a NULL dropped these rows”.
3 · X-ray any query
Watch rows join, get filtered, piled into groups and ranked, step by step. Tap a row to see where it came from.