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Aporeon

Technical interview practice, from one developer to another.

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SQL Practice

Hands-on SQL query problems for backend data access, reporting, concurrency, and performance.

18 drillsPractice one at a timeOrdered pathCoding problem

Recommended start

Paid Order Totals

Writes a left-join aggregate that keeps active customers with no paid orders and applies date filters without changing join semantics.

Easy~15 minStep 1 of 18

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1Paid Order TotalsRecommended startWrites a left-join aggregate that keeps active customers with no paid orders and applies date filters without changing join semantics.Coding problemEasy~15 minSQLSQL ServerPractice 2Users Without OrdersUses NOT EXISTS to express an anti-join without null traps or duplicate parent rows.Coding problemEasy~15 minSQLSQL ServerPractice 3Match Every TagSolves a many-to-many all-match filter with grouping and a deliberate empty-selection rule.Coding problemMedium~25 minSQLSQL ServerPractice 4Booking OverlapImplements correct half-open interval overlap logic for booking validation.Coding problemMedium~25 minSQLSQL ServerPractice 5Filter Without DuplicatesUses EXISTS instead of a row-multiplying join when the related table is only a filter.Coding problemEasy~15 minSQLSQL ServerPractice 6Daily Order KPIsBuilds several daily business metrics in one grouped query while keeping each metric definition explicit.Coding problemMedium~25 minSQLSQL ServerPractice 7Duplicate EmailsGroups normalized email values to identify duplicates before adding or repairing a uniqueness rule.Coding problemEasy~15 minSQLSQL ServerPractice 8Avoid Fanout TotalsPre-aggregates two one-to-many relationships before joining them to prevent fanout from corrupting financial totals.Coding problemMedium~25 minSQLSQL ServerPractice 9Rolling RevenueCombines daily aggregation, a calendar table, and a window frame to calculate true seven-calendar-day revenue.Coding problemHard~35 minSQLSQL ServerPractice 10Latest Order StatusUses ROW_NUMBER with a deterministic tie-breaker to return the complete latest status row per order.Coding problemMedium~25 minSQLSQL ServerPractice 11Top Products Per CategoryAggregates product sales and ranks independently inside each category with a deterministic top-N rule.Coding problemMedium~25 minSQLSQL ServerPractice 12Keyset Order PageImplements descending keyset pagination with a compound cursor and stable ordering.Coding problemMedium~25 minSQLSQL ServerPractice 13Running BalanceUses an ordered window aggregate to calculate a deterministic running balance per account.Coding problemMedium~25 minSQLSQL ServerPractice 14Longest Login StreakUses the gaps-and-islands pattern to find consecutive login-day streaks after deduplicating same-day activity.Coding problemHard~35 minSQLSQL ServerPractice 15Category Tree PathsUses a recursive CTE to return hierarchy depth and a readable root-to-node path.Coding problemHard~35 minSQLSQL ServerPractice 16Local Date, UTC RangeConverts local-day boundaries to UTC once, preserving correct daylight-saving behavior and index use on stored UTC timestamps.Coding problemHard~35 minSQLSQL ServerPractice 17Atomic Inventory ReserveUses a conditional update and transaction to prevent overselling while recording the reservation consistently.Coding problemMedium~25 minSQLSQL ServerPractice 18Deduplicate ImportsRanks duplicate import rows, previews the deletion set, and removes older copies with deterministic retention.Coding problemMedium~25 minSQLSQL ServerPractice

Practice order

18 drillsOne at a time

1Paid Order TotalsUp next / Easy / ~15 min
2Users Without OrdersEasy / ~15 min
3Match Every TagMedium / ~25 min
4Booking OverlapMedium / ~25 min
5Filter Without DuplicatesEasy / ~15 min
6Daily Order KPIsMedium / ~25 min
7Duplicate EmailsEasy / ~15 min
8Avoid Fanout TotalsMedium / ~25 min

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