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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.

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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🔒