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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

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

#1Second-Highest SalarySubqueriesMediumFree #2Nth-Highest Salary (with DENSE_RANK)Window functionsMediumFree #4Highest-Paid Employee in Each DepartmentWindow functionsMediumFree Unlock 48 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🔒#14Delete Duplicate RowsChanging dataMedium🔒#17Month-over-Month Growth (LAG)Window functionsMedium🔒#20Each Customer's Second OrderWindow functionsMedium🔒#213-Order Moving AverageWindow functionsMedium🔒#25Pivot Order Statuses into ColumnsCASE & NULLMedium🔒#27Each Category's Share of RevenueWindow functionsMedium🔒#36Subquery vs JOIN vs CTESubqueriesMedium🔒#37Employees Paid Above Their Department AverageSubqueriesMedium🔒#38EXISTS vs INSubqueriesMedium🔒#39How do indexes work?Indexes & transactionsMedium🔒#41Normalization (1NF, 2NF, 3NF)Schema designMedium🔒#42ACID transactionsIndexes & transactionsMedium🔒#44SQL injection and how to prevent itIndexes & transactionsMedium🔒#47SQL's logical order of executionBasicsMedium🔒#48Pagination (LIMIT/OFFSET vs keyset)Sorting & limitsMedium🔒#50Days Between a Customer's OrdersWindow functionsMedium🔒#53Split Employees into Salary QuartilesWindow functionsMedium🔒#54Cancellation Rate per CustomerCASE & NULLMedium🔒#58Upsert (insert or update)Changing dataMedium🔒#59Update Using Another TableChanging dataMedium🔒#60Design a Table with ConstraintsSchema designMedium🔒#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🔒#75Each Customer's Most Recent OrderWindow functionsMedium🔒#78Salary Gap to the Department's Top EarnerWindow functionsMedium🔒#80Revenue by Category and MonthGroup byMedium🔒#82Replication vs shardingIndexes & transactionsMedium🔒#84Schema migrations in productionSchema designMedium🔒#85Choosing column types (money, time, IDs)Schema designMedium🔒#86JSON columns in a relational databaseSchema designMedium🔒#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🔒#94INTERSECT and EXCEPTBasicsMedium🔒#95Row With the Max Value per Group (without window functions)JoinsMedium🔒#98Combine Rows into a List (GROUP_CONCAT / STRING_AGG)Group byMedium🔒#99Aggregate of AggregatesSubqueriesMedium🔒