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🗄 Interview Prep

SQL & Databases

Joins, GROUP BY, subqueries, CTEs and window functions, run on real tables in your browser. Asked for back-end, data and full-stack roles.

📘 Learn it from scratch

Visual, hands-on lessons for every pattern in this topic. Do the lesson, then come back and the questions click.

Tables & SELECT QueriesFiltering Rows with WHERESorting, Limiting & DISTINCTCounting & Summarizing (Aggregates)Grouping Results with GROUP BYCombining Tables with JOINSubqueries & CTEsINSERT, UPDATE & DELETEDesigning & Creating Tables

100 questions, most asked first

Ordered so the most useful questions for learning each pattern come first.

Any difficulty Easy Medium Hard

All patterns Subqueries Window functions Joins Group by Self joins & CTEs Changing data Dates & strings CASE & NULL Sorting & limits Aggregates Filtering Basics Indexes & transactions Schema design

#8Top 3 Salaries in Each DepartmentWindow functionsHardFree Unlock 9 more SQL & Databases questions · all 1,200 questions for $39/year (about 11¢ a day)See full access #15Users With 3+ Consecutive Login DaysWindow functionsHard🔒#19Customers Who Bought From Every CategoryGroup byHard🔒#29Next-Day RetentionSelf joins & CTEsHard🔒#32Org Chart with a Recursive CTESelf joins & CTEsHard🔒#43Transaction isolation levelsIndexes & transactionsHard🔒#51Median SalaryWindow functionsHard🔒#56Customers Who Ordered in Consecutive MonthsSelf joins & CTEsHard🔒#81Optimistic vs pessimistic lockingIndexes & transactionsHard🔒#96Orders by Signup CohortGroup byHard🔒