Free lessons

SQL Lessons

Every lesson in the SQL practice course, free to read. Each skill has a lesson that starts simple and gets technical, and a quick revise page. Practice and progress tracking need a free account.

Start free About the course

SQL Fundamentals

Selecting, filtering, joining and grouping rows - and the NULL, duplicate and fan-out rules that decide whether a query that runs is a query that's right.

Analytical SQL

Window functions, CTEs and recursive queries, and time series - top-N per group, running totals, period-over-period change, retention and streaks, the questions data-heavy interviews and dashboards ask.

Window Functions: Ranking and Top-N

Ranking and Top-N per GroupROW_NUMBER, RANK and DENSE_RANK with PARTITION BY: the top N per category, how ties change the answer, and keeping one row per duplicate.

Window Functions: Running Totals and Comparisons

Running Totals, LAG/LEAD and FramesCumulative sums, moving averages over a window frame, change versus the previous period, and each row's share of its group.

CTEs and Recursive Queries

CTEs and Recursive QueriesNaming query steps with WITH, and WITH RECURSIVE for org charts, category trees and series - with a guard against cycles.

Dates, Time Series, Gaps and Islands

Dates, Time Series, Gaps and IslandsBucketing timestamps with date_trunc, filling empty days with generate_series, time zones, retention, and streaks as gaps and islands.

SQL for Backend Engineers

Designing tables the database can defend, changing them on live data, writing safely under concurrency with upserts, MERGE and transactions, making queries fast with the right index, and storing JSON without losing SQL.