Master the SQL that interviews actually test

A LeetCode-style gym of 101+ problems spanning every corner of SQL — joins, aggregation, subqueries, set ops, CTEs & recursion, window functions, ranking, pivoting, dates, strings and NULL logic. Every query runs live in your browser on a real SQLite engine, and each ships a worked solution with a step-by-step explanation.

101 problems

#	Title	Category	Difficulty
1	Generate a Number Sequence WITH RECURSIVEanchor + recursive memberRecursive CTEs	Recursive CTEs	Easy
2	Running Total of Sales SUM() OVERORDER BY frameWindow Functions	Window Functions	Easy
3	Above-Average Spenders multiple CTEsaggregate then compareCTEs	CTEs	Medium
4	Second Highest Salary (DENSE_RANK) DENSE_RANKfilter on rankRanking	Ranking	Medium
5	Customers Who Bought Every Product relational divisionCOUNT(DISTINCT)HAVINGAdvanced Filtering	Advanced Filtering	Hard
6	Custom Priority Sort CASE in ORDER BYcustom collation orderOrdering	Ordering	Medium
7	Employee Hierarchy Levels WITH RECURSIVEhierarchylevel counterRecursive CTEs	Recursive CTEs	Medium
8	3-Day Moving Average AVG() OVERROWS BETWEEN frameWindow Functions	Window Functions	Medium
9	Filter, Then Compare Within a CTE CTE filteringself-reference for aggregateCTEs	CTEs	Medium
10	Top 2 Products per Category ROW_NUMBERPARTITION BYtop-N-per-groupRanking	Ranking	Medium
11	Find Duplicate Emails GROUP BYHAVING COUNTAdvanced Filtering	Advanced Filtering	Easy
12	Sort With NULLs Last NULL orderingexpression sort keyOrdering	Ordering	Medium
13	Build the Org Path WITH RECURSIVEstring accumulationpathRecursive CTEs	Recursive CTEs	Medium
14	Month-over-Month Change LAG()previous-row accessWindow Functions	Window Functions	Medium
15	Category Revenue Leaders chained CTEsjoin CTE to CTEratio thresholdCTEs	CTEs	Hard
16	RANK vs DENSE_RANK RANKDENSE_RANKtie handlingRanking	Ranking	Easy
17	Second Highest, No LIMIT correlated subqueryMAX of a filtered setAdvanced Filtering	Advanced Filtering	Medium
18	Order by Best Value computed sort keyCASTalias in ORDER BYOrdering	Ordering	Easy
19	Fill the Date Gaps WITH RECURSIVEcalendar generationLEFT JOINRecursive CTEs	Recursive CTEs	Hard
20	Percent of Category Total SUM() OVER PARTITION BYratio to group totalWindow Functions	Window Functions	Medium
21	Highest and Lowest Earner ORDER BY + LIMIT inside a CTEUNION ALLfinal orderingCTEs	CTEs	Medium
22	Consecutive Login Streaks (Gaps & Islands) ROW_NUMBERgaps and islandsgrouping trickRanking	Ranking	Hard
23	Customers With No Orders anti-joinNOT EXISTSAdvanced Filtering	Advanced Filtering	Easy
24	Multi-Key Ordering multiple sort keysmixed ASC/DESCOrdering	Ordering	Easy
25	Fibonacci Sequence WITH RECURSIVEcarried statemultiple accumulatorsRecursive CTEs	Recursive CTEs	Medium
26	First and Last Close per Symbol FIRST_VALUELAST_VALUEexplicit frame gotchaWindow Functions	Window Functions	Hard
27	Score Quartiles with NTILE NTILEbucketingRanking	Ranking	Medium
28	Departments Above Average Headcount aggregate of aggregatessubquery in WHEREAdvanced Filtering	Advanced Filtering	Medium
29	Difference From Subject Average AVG() OVER PARTITION BYrow vs group comparisonWindow Functions	Window Functions	Medium
30	Employee and Their Manager INNER JOINself-joinJoins	Joins	Easy
31	Every Product and Units Sold LEFT JOINCOALESCEGROUP BYJoins	Joins	Easy
32	Headcount per Department INNER JOINGROUP BYCOUNTJoins	Joins	Easy
33	Assign Letter Grades (Band Join) non-equi joinBETWEENrange lookupJoins	Joins	Medium
34	Customers Who Placed Orders semi-joinEXISTSINJoins	Joins	Medium
35	Customers Without Orders anti-joinNOT EXISTSLEFT JOIN ... IS NULLJoins	Joins	Medium
36	Size × Color Product Matrix CROSS JOINCartesian productJoins	Joins	Medium
37	Warmer Than the Previous Day self-joinnon-equi joindate arithmeticJoins	Joins	Medium
38	All Students and All Memberships FULL OUTER JOINNULLS LASTunmatched rowsJoins	Joins	Hard
39	Coworker Pairs in the Same Department self-joinpair generationdeduplicate A-B/B-AJoins	Joins	Hard
40	Spend per Customer Across Three Tables three-table joinGROUP BYSUMJoins	Joins	Hard
41	Distinct Products Per Store COUNT(DISTINCT)GROUP BYAggregation	Aggregation	Easy
42	Orders Per Customer GROUP BYCOUNTAggregation	Aggregation	Easy
43	Revenue By Category GROUP BYSUMAVGAggregation	Aggregation	Easy
44	Big Spenders (HAVING Threshold) GROUP BYHAVINGSUMAggregation	Aggregation	Medium
45	Grade Distribution By Class GROUP BY multiple columnsCOUNTAggregation	Aggregation	Medium
46	Paid vs Unpaid Invoices conditional aggregationCASEFILTERAggregation	Aggregation	Medium
47	Pass Rate Per Quiz ratio of subgroup to totalconditional aggregationAVGAggregation	Aggregation	Medium
48	Tags Per Post (Ordered List) group_concatordered aggregationGROUP BYAggregation	Aggregation	Medium
49	Department Payroll (Aggregate Over Join) aggregate over joinJOINGROUP BYAggregation	Aggregation	Hard
50	Favorite Genre Per User (Mode) modemost-frequent valueCOUNTAggregation	Aggregation	Hard
51	Weighted Course GPA weighted averageSUMROUNDAggregation	Aggregation	Hard
52	Order Count Per Customer (Scalar Subquery) scalar subquerysubquery in SELECTcorrelated subquerySubqueries	Subqueries	Easy
53	Orders Above the Overall Average comparison to scalar subqueryAVGsubquery in WHERESubqueries	Subqueries	Easy
54	Books by Prolific Authors (Uncorrelated Subquery Filter) uncorrelated subqueryINGROUP BYSubqueries	Subqueries	Medium
55	Customers With a Big Order (IN vs EXISTS) INEXISTSsemi-joinSubqueries	Subqueries	Medium
56	Products Priced Above Their Category Average correlated subquerysubquery in WHEREAVGSubqueries	Subqueries	Medium
57	Top Spenders via a Derived Table derived tablesubquery in FROMGROUP BYSubqueries	Subqueries	Medium
58	Departments Beating the Company Average (HAVING + Subquery) subquery in HAVINGAVGGROUP BYSubqueries	Subqueries	Hard
59	Each Customer's Most Expensive Order greatest-n-per-groupcorrelated subqueryMAXSubqueries	Subqueries	Hard
60	All People Across Two Tables (UNION dedupe) UNIONdeduplicationORDER BYSet Operations	Set Operations	Easy
61	Customers Who Bought on Both Channels (INTERSECT) INTERSECTdeduplicationORDER BYSet Operations	Set Operations	Easy
62	Churned Customers (EXCEPT) EXCEPTanti-joindeduplicationSet Operations	Set Operations	Medium
63	UNION ALL vs UNION: Counting Logins UNION ALLUNIONduplicatesSet Operations	Set Operations	Medium
64	Items on Exactly One Menu (Symmetric Difference) symmetric differenceEXCEPTUNIONSet Operations	Set Operations	Hard
65	Signup Funnel Conversion CTE chainingFunnelConversion rateCTEs	CTEs	Medium
66	Everyone Who Reports Up To A Manager Recursive CTETransitive ClosureGraphRecursive CTEs	Recursive CTEs	Hard
67	Split CSV Tags Into Rows Recursive CTEString SplitSUBSTRRecursive CTEs	Recursive CTEs	Hard
68	Exam Score Percentile CUME_DISTPERCENT_RANKWindow FunctionsWindow Functions	Window Functions	Medium
69	Gap To Next Login LEADWindow FunctionsDate DiffWindow Functions	Window Functions	Medium
70	Running Maximum Balance MAX OVERRunning AggregateWindow FunctionsWindow Functions	Window Functions	Medium
71	Daily Cumulative Sales (RANGE vs ROWS) RANGE frameROWS frameWindow FunctionsWindow Functions	Window Functions	Hard
72	Deduplicate Keeping Latest Contact ROW_NUMBERDeduplicationRankingRanking	Ranking	Medium
73	Top Scorers With Ties RANKTop-N with tiesRankingRanking	Ranking	Medium
74	Median Home Price ROW_NUMBERMedianRankingRanking	Ranking	Hard
75	Pivot Order Status Counts conditional aggregationCOUNTSUMPivoting	Pivoting	Medium
76	Pivot Quarterly Sales into Columns conditional aggregationSUMCASEPivoting	Pivoting	Medium
77	Unpivot Subject Scores into Rows UNION ALLunpivotPivoting	Pivoting	Medium
78	Binary Skill Presence Matrix conditional aggregationMAXCASEPivoting	Pivoting	Hard
79	Pivot Months then Add a Total conditional aggregationSUMderived columnPivoting	Pivoting	Hard
80	Two-Dimensional Pivot: Store by Quarter conditional aggregationtwo-dimensional pivotSUMPivoting	Pivoting	Hard
81	Days Between Subscription Dates juliandayDates & Time	Dates & Time	Easy
82	Extract Year, Month, and Weekday strftimeDates & Time	Dates & Time	Easy
83	Weekend Events Only strftimeweekday filterDates & Time	Dates & Time	Easy
84	Age in Whole Years strftimewhole-year ageDates & Time	Dates & Time	Medium
85	Count Events Per Month strftimeGROUP BY monthDates & Time	Dates & Time	Medium
86	First and Last Day of Each Month date modifiersstart of monthDates & Time	Dates & Time	Medium
87	Orders in the Last 30 Days date arithmeticreference dateDates & Time	Dates & Time	Medium
88	Events Per Weekday Name strftimeweekday mappingGROUP BYDates & Time	Dates & Time	Hard
89	Build a Full Name concatenation\|\|String Manipulation	String Manipulation	Easy
90	Case-Insensitive Product Search lowerLIKEString Manipulation	String Manipulation	Easy
91	Clean Phone Numbers with replace replaceString Manipulation	String Manipulation	Easy
92	Count Items in a CSV Field lengthreplaceString Manipulation	String Manipulation	Medium
93	Extract the Email Domain substrinstrString Manipulation	String Manipulation	Medium
94	Format Prices with printf printfString Manipulation	String Manipulation	Medium
95	Avoid Divide-by-Zero with NULLIF NULLIFCOALESCENULL Handling	NULL Handling	Easy
96	Default Missing Values with COALESCE COALESCEIFNULLNULL Handling	NULL Handling	Easy
97	COUNT() vs COUNT(column) and AVG over NULLs COUNT()COUNT(col)AVG ignores NULLNULL Handling	NULL Handling	Medium
98	IS NULL, IS NOT NULL, and Three-Valued Logic IS NULLIS NOT NULLthree-valued logicNULL Handling	NULL Handling	Medium
99	The NOT IN NULL Trap NOT EXISTSNOT IN pitfallthree-valued logicNULL Handling	NULL Handling	Hard
100	Bought Product X But Not Product Y EXISTSNOT EXISTSAnti-joinAdvanced Filtering	Advanced Filtering	Medium
101	Case-Insensitive Name Sort COLLATE NOCASEgroup_concatDeterministic orderOrdering	Ordering	Medium