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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.

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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

#1Second-Highest SalarySubqueriesMediumFree #2Nth-Highest Salary (with DENSE_RANK)Window functionsMediumFree #3Customers Who Never OrderedJoinsEasyFree #4Highest-Paid Employee in Each DepartmentWindow functionsMediumFree #5Find Duplicate EmailsGroup byEasyFree #6Employees Earning More Than Their ManagersSelf joins & CTEsEasyFree #7Average Salary by DepartmentGroup byEasyFree #8Top 3 Salaries in Each DepartmentWindow functionsHardFree #9INNER vs LEFT vs RIGHT vs FULL JOINJoinsEasyFree #10WHERE vs HAVINGGroup byEasyFree Unlock 90 more SQL & Databases questions · all 1,200 questions for $39/year (about 11¢ a day)See full access #11Rank ScoresWindow functionsMedium🔒#12Running Total of RevenueWindow functionsMedium🔒#13Number of Orders per Customer (Including Zero)JoinsEasy🔒#14Delete Duplicate RowsChanging dataMedium🔒#15Users With 3+ Consecutive Login DaysWindow functionsHard🔒#16Monthly RevenueDates & stringsEasy🔒#17Month-over-Month Growth (LAG)Window functionsMedium🔒#18Products Never OrderedSubqueriesEasy🔒#19Customers Who Bought From Every CategoryGroup byHard🔒#20Each Customer's Second OrderWindow functionsMedium🔒#213-Order Moving AverageWindow functionsMedium🔒#22Employees Hired in a Date RangeDates & stringsEasy🔒#23Handling NULLs with COALESCECASE & NULLEasy🔒#24Salary Bands with CASECASE & NULLEasy🔒#25Pivot Order Statuses into ColumnsCASE & NULLMedium🔒#26Top Customer by Total SpendSorting & limitsEasy🔒#27Each Category's Share of RevenueWindow functionsMedium🔒#28Each Customer's First Order DateAggregatesEasy🔒#29Next-Day RetentionSelf joins & CTEsHard🔒#30Daily Active UsersAggregatesEasy🔒#31Who Referred the Most Customers?Self joins & CTEsEasy🔒#32Org Chart with a Recursive CTESelf joins & CTEsHard🔒#33Departments With No EmployeesJoinsEasy🔒#34Employees Without a DepartmentFilteringEasy🔒#35UNION vs UNION ALLBasicsEasy🔒#36Subquery vs JOIN vs CTESubqueriesMedium🔒#37Employees Paid Above Their Department AverageSubqueriesMedium🔒#38EXISTS vs INSubqueriesMedium🔒#39How do indexes work?Indexes & transactionsMedium🔒#40Primary keys and foreign keysSchema designEasy🔒#41Normalization (1NF, 2NF, 3NF)Schema designMedium🔒#42ACID transactionsIndexes & transactionsMedium🔒#43Transaction isolation levelsIndexes & transactionsHard🔒#44SQL injection and how to prevent itIndexes & transactionsMedium🔒#45COUNT(*) vs COUNT(column) vs COUNT(DISTINCT)AggregatesEasy🔒#46DISTINCT vs GROUP BYGroup byEasy🔒#47SQL's logical order of executionBasicsMedium🔒#48Pagination (LIMIT/OFFSET vs keyset)Sorting & limitsMedium🔒#49Extract email domainsDates & stringsEasy🔒#50Days Between a Customer's OrdersWindow functionsMedium🔒#51Median SalaryWindow functionsHard🔒#52The Most Popular ProductAggregatesEasy🔒#53Split Employees into Salary QuartilesWindow functionsMedium🔒#54Cancellation Rate per CustomerCASE & NULLMedium🔒#55Average Order Value by CityAggregatesEasy🔒#56Customers Who Ordered in Consecutive MonthsSelf joins & CTEsHard🔒#57DELETE vs TRUNCATE vs DROPChanging dataEasy🔒#58Upsert (insert or update)Changing dataMedium🔒#59Update Using Another TableChanging dataMedium🔒#60Design a Table with ConstraintsSchema designMedium🔒#61What is a view?Schema designEasy🔒#62Stored procedures and triggersSchema designMedium🔒#63OLTP vs OLAPIndexes & transactionsMedium🔒#64SQL vs NoSQLSchema designMedium🔒#65When to denormalizeSchema designMedium🔒#66How would you speed up a slow query?Indexes & transactionsMedium🔒#67The N+1 query problemJoinsMedium🔒#68What are window functions?Window functionsMedium🔒#69CTE vs subquery vs temporary tableSelf joins & CTEsMedium🔒#70Find Gaps in a SequenceWindow functionsMedium🔒#71Best-Selling Product in Each CategoryWindow functionsMedium🔒#72Customers Who Never CancelledGroup byEasy🔒#73City With the Most CustomersGroup byEasy🔒#74Orders Above the Average AmountSubqueriesEasy🔒#75Each Customer's Most Recent OrderWindow functionsMedium🔒#76Units Sold per Product (Including Zero)JoinsEasy🔒#77Hires per YearDates & stringsEasy🔒#78Salary Gap to the Department's Top EarnerWindow functionsMedium🔒#79Same-Day Duplicate EventsGroup byEasy🔒#80Revenue by Category and MonthGroup byMedium🔒#81Optimistic vs pessimistic lockingIndexes & transactionsHard🔒#82Replication vs shardingIndexes & transactionsMedium🔒#83Soft deletesSchema designEasy🔒#84Schema migrations in productionSchema designMedium🔒#85Choosing column types (money, time, IDs)Schema designMedium🔒#86JSON columns in a relational databaseSchema designMedium🔒#87Searching text with LIKEFilteringEasy🔒#88Filtering with IN and BETWEENFilteringEasy🔒#89Case-Insensitive Duplicate EmailsDates & stringsMedium🔒#90Cumulative Customer SignupsWindow functionsMedium🔒#91Subtotals with a Grand Total RowGroup byMedium🔒#92Pairs of Customers in the Same CitySelf joins & CTEsMedium🔒#93Every Combination with CROSS JOINJoinsEasy🔒#94INTERSECT and EXCEPTBasicsMedium🔒#95Row With the Max Value per Group (without window functions)JoinsMedium🔒#96Orders by Signup CohortGroup byHard🔒#97Copy Rows with INSERT … SELECTChanging dataEasy🔒#98Combine Rows into a List (GROUP_CONCAT / STRING_AGG)Group byMedium🔒#99Aggregate of AggregatesSubqueriesMedium🔒#100How to approach any SQL interview questionBasicsEasy🔒