Skip to content

🗄 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

Ranked by how often each question comes up in real interviews. #1 is the most common.

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🔒