A focused path from what a table is to why a query is slow. Built for someone who wants to say, with a straight face, that they are proficient in SQL: joins, aggregation, window functions, and the index reasoning that separates a scripter from an engineer.
Tables, keys, types. SELECT / WHERE / ORDER BY, the NULL trap, and the logical order every query runs in.
Start hereINNER, LEFT, RIGHT, FULL, CROSS, and self-joins. Anti-joins, NULLs, and the WHERE-vs-ON outer-join trap.
Open moduleCOUNT / SUM / AVG, GROUP BY mechanics, HAVING vs WHERE, and how NULLs behave inside aggregates.
Open moduleScalar & correlated subqueries, EXISTS, the NOT IN + NULL trap, CTEs, and recursive org-chart queries.
Open moduleOVER / PARTITION BY, ranking, top-N-per-group, LAG/LEAD, running totals, and the ROWS-vs-RANGE frame.
Open moduleB-tree indexes, EXPLAIN ANALYZE, sargability, composite indexes, and a workflow for debugging slow queries.
Open moduleEvery module runs on the same tiny company database, so you focus on technique, not on re-reading a new schema each time. People work in departments, some manage others, and they place orders.