500+ real-world questions across topics, roles and industries. Free, forever.
Return the second-highest salary from an employees table without using LIMIT or TOP.
Build monthly signup cohorts and compute their 90-day churn rate for a subscription business.
Compute a 7-day rolling retention curve for a SaaS product using window functions and self-joins.
Drill down until you own it.
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Curated sets by role and industry. Timed. Solutions revealed after you attempt.
Shipping through Q3 and Q4 2026.
Postgres + DuckDB, right in the page.
Nudges without spoiling the answer.
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