Crack the SQL Interview, From Fundamentals to Window Functions
SQL is the single most tested skill in data, analytics, and backend interviews — yet most candidates stumble on the same traps: silent NULL behavior, joins that multiply rows, and the window-function questions that now show up at almost every company. This track turns interview SQL into a repeatable skill. You work through the exact question patterns asked at product and data-heavy companies, writing and running real queries against sample tables until each pattern becomes second nature.
What You Will Learn
You start with the fundamentals interviewers screen on — SELECT, column expressions, WHERE filtering gotchas, ORDER BY with LIMIT, and aggregate functions. From there you master every join type and the GROUP BY / HAVING traps that catch most candidates, plus correct NULL handling. The core of the track is the material that decides offers: subqueries and correlated subqueries, CTEs and recursive CTEs, and window functions including ROW_NUMBER, RANK, LAG/LEAD, running totals, top-N per group, and the classic Nth-highest-salary puzzles. Advanced courses cover gaps and islands, streaks, pivoting, cohort and retention analysis, funnel and A/B-test queries, query optimization with EXPLAIN, indexing strategy, transactions and isolation levels, and schema design with full mock interviews.
The Learning Path
Thirty courses progress from A1 through B2. The first two are free and cover SQL fundamentals and SELECT basics. The A1–A2 courses build the foundation — filtering, sorting, aggregation, every join type, grouping, and NULL handling. The ten B1 courses move into mid-level interview territory: subqueries, CTEs, and the full window-function toolkit modern interviews rely on. The ten B2 courses tackle senior-level analytical SQL — gaps and islands, streak and cohort analysis, funnel and A/B-test queries, performance tuning, indexing, transactions, and schema design, closing with full mock interview sessions.
How It Works
Each course is split into short, hands-on lessons you complete in the built-in editor with real-time feedback and an AI tutor on hand when you get stuck. You write and run actual queries against realistic tables — not multiple-choice trivia — so every lesson rehearses the way you will be asked to think out loud in the real interview.