🗄 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.
100 questions, most asked first
Ordered so the most useful questions for learning each pattern come first.
All patternsSubqueriesWindow functionsJoinsGroup bySelf joins & CTEsChanging dataDates & stringsCASE & NULLSorting & limitsAggregatesFilteringBasicsIndexes & transactionsSchema design
#1🗄Second-Highest SalarySubqueriesMediumFree#2🗄Nth-Highest Salary (with DENSE_RANK)Window functionsMediumFree#3🗄Customers Who Never OrderedJoinsEasyFree#4🗄Highest-Paid Employee in Each DepartmentWindow functionsMediumFree#5🗄Find Duplicate EmailsGroup byEasyFree#6🗄Employees Earning More Than Their ManagersSelf joins & CTEsEasyFree#7🗄Average Salary by DepartmentGroup byEasyFree#8🗄Top 3 Salaries in Each DepartmentWindow functionsHardFree#9💬INNER vs LEFT vs RIGHT vs FULL JOINJoinsEasyFree#10🗄WHERE vs HAVINGGroup byEasyFree🔓Unlock 90 more SQL & Databases questions · all 1,200 questions for $39/year (about 11¢ a day)See full access#11🗄Rank ScoresWindow functionsMedium🔒#12🗄Running Total of RevenueWindow functionsMedium🔒#13🗄Number of Orders per Customer (Including Zero)JoinsEasy🔒#14🗄Delete Duplicate RowsChanging dataMedium🔒#15🗄Users With 3+ Consecutive Login DaysWindow functionsHard🔒#16🗄Monthly RevenueDates & stringsEasy🔒#17🗄Month-over-Month Growth (LAG)Window functionsMedium🔒#18🗄Products Never OrderedSubqueriesEasy🔒#19🗄Customers Who Bought From Every CategoryGroup byHard🔒#20🗄Each Customer's Second OrderWindow functionsMedium🔒#21🗄3-Order Moving AverageWindow functionsMedium🔒#22🗄Employees Hired in a Date RangeDates & stringsEasy🔒#23🗄Handling NULLs with COALESCECASE & NULLEasy🔒#24🗄Salary Bands with CASECASE & NULLEasy🔒#25🗄Pivot Order Statuses into ColumnsCASE & NULLMedium🔒#26🗄Top Customer by Total SpendSorting & limitsEasy🔒#27🗄Each Category's Share of RevenueWindow functionsMedium🔒#28🗄Each Customer's First Order DateAggregatesEasy🔒#29🗄Next-Day RetentionSelf joins & CTEsHard🔒#30🗄Daily Active UsersAggregatesEasy🔒#31🗄Who Referred the Most Customers?Self joins & CTEsEasy🔒#32🗄Org Chart with a Recursive CTESelf joins & CTEsHard🔒#33🗄Departments With No EmployeesJoinsEasy🔒#34🗄Employees Without a DepartmentFilteringEasy🔒#35🗄UNION vs UNION ALLBasicsEasy🔒#36💬Subquery vs JOIN vs CTESubqueriesMedium🔒#37🗄Employees Paid Above Their Department AverageSubqueriesMedium🔒#38💬EXISTS vs INSubqueriesMedium🔒#39💬How do indexes work?Indexes & transactionsMedium🔒#40💬Primary keys and foreign keysSchema designEasy🔒#41💬Normalization (1NF, 2NF, 3NF)Schema designMedium🔒#42💬ACID transactionsIndexes & transactionsMedium🔒#43💬Transaction isolation levelsIndexes & transactionsHard🔒#44💬SQL injection and how to prevent itIndexes & transactionsMedium🔒#45🗄COUNT(*) vs COUNT(column) vs COUNT(DISTINCT)AggregatesEasy🔒#46🗄DISTINCT vs GROUP BYGroup byEasy🔒#47💬SQL's logical order of executionBasicsMedium🔒#48🗄Pagination (LIMIT/OFFSET vs keyset)Sorting & limitsMedium🔒#49🗄Extract email domainsDates & stringsEasy🔒#50🗄Days Between a Customer's OrdersWindow functionsMedium🔒#51🗄Median SalaryWindow functionsHard🔒#52🗄The Most Popular ProductAggregatesEasy🔒#53🗄Split Employees into Salary QuartilesWindow functionsMedium🔒#54🗄Cancellation Rate per CustomerCASE & NULLMedium🔒#55🗄Average Order Value by CityAggregatesEasy🔒#56🗄Customers Who Ordered in Consecutive MonthsSelf joins & CTEsHard🔒#57💬DELETE vs TRUNCATE vs DROPChanging dataEasy🔒#58🗄Upsert (insert or update)Changing dataMedium🔒#59🗄Update Using Another TableChanging dataMedium🔒#60🗄Design a Table with ConstraintsSchema designMedium🔒#61💬What is a view?Schema designEasy🔒#62💬Stored procedures and triggersSchema designMedium🔒#63💬OLTP vs OLAPIndexes & transactionsMedium🔒#64💬SQL vs NoSQLSchema designMedium🔒#65💬When to denormalizeSchema designMedium🔒#66💬How would you speed up a slow query?Indexes & transactionsMedium🔒#67💬The N+1 query problemJoinsMedium🔒#68💬What are window functions?Window functionsMedium🔒#69💬CTE vs subquery vs temporary tableSelf joins & CTEsMedium🔒#70🗄Find Gaps in a SequenceWindow functionsMedium🔒#71🗄Best-Selling Product in Each CategoryWindow functionsMedium🔒#72🗄Customers Who Never CancelledGroup byEasy🔒#73🗄City With the Most CustomersGroup byEasy🔒#74🗄Orders Above the Average AmountSubqueriesEasy🔒#75🗄Each Customer's Most Recent OrderWindow functionsMedium🔒#76🗄Units Sold per Product (Including Zero)JoinsEasy🔒#77🗄Hires per YearDates & stringsEasy🔒#78🗄Salary Gap to the Department's Top EarnerWindow functionsMedium🔒#79🗄Same-Day Duplicate EventsGroup byEasy🔒#80🗄Revenue by Category and MonthGroup byMedium🔒#81💬Optimistic vs pessimistic lockingIndexes & transactionsHard🔒#82💬Replication vs shardingIndexes & transactionsMedium🔒#83💬Soft deletesSchema designEasy🔒#84💬Schema migrations in productionSchema designMedium🔒#85💬Choosing column types (money, time, IDs)Schema designMedium🔒#86🗄JSON columns in a relational databaseSchema designMedium🔒#87🗄Searching text with LIKEFilteringEasy🔒#88🗄Filtering with IN and BETWEENFilteringEasy🔒#89🗄Case-Insensitive Duplicate EmailsDates & stringsMedium🔒#90🗄Cumulative Customer SignupsWindow functionsMedium🔒#91🗄Subtotals with a Grand Total RowGroup byMedium🔒#92🗄Pairs of Customers in the Same CitySelf joins & CTEsMedium🔒#93🗄Every Combination with CROSS JOINJoinsEasy🔒#94🗄INTERSECT and EXCEPTBasicsMedium🔒#95🗄Row With the Max Value per Group (without window functions)JoinsMedium🔒#96🗄Orders by Signup CohortGroup byHard🔒#97🗄Copy Rows with INSERT … SELECTChanging dataEasy🔒#98🗄Combine Rows into a List (GROUP_CONCAT / STRING_AGG)Group byMedium🔒#99🗄Aggregate of AggregatesSubqueriesMedium🔒#100💬How to approach any SQL interview questionBasicsEasy🔒