SQL lessons

Querying, joining, window functions and the analyst interview. 6 lessons across 5 tracks, each ending with a short self-check.

6lessons

60mtotal

5tracks

12checks

Query basics

SELECT, WHERE, ORDER BY and how a query is actually executed.

1 · 9m

SELECT, WHERE and logical orderThe clauses, and the order the database actually evaluates them in.beginner9m

Joins & sets

Combining tables without silently changing your row count.

1 · 11m

JoinsEvery join type, the fan-out trap, and why WHERE breaks a LEFT JOIN.intermediate11m

Aggregation

GROUP BY, HAVING, and the mistakes that produce wrong totals.

1 · 9m

GROUP BY and HAVINGAggregating correctly, and the WHERE/HAVING distinction interviewers ask about.beginner9m

Window functions

Ranking, running totals and period-over-period.

1 · 12m

Window functionsRanking, running totals and period comparisons without collapsing rows.advanced12m

Structure & performance

Subqueries, CTEs, indexes and reading a plan.

2 · 19m

CTEs and subqueriesStructuring a long query, correlated subqueries, and recursion.intermediate9m Indexes and reading a planWhy a query is slow, and what an index can and cannot fix.advanced10m