Easy
18 original prompts with hidden variants and debrief notes.
Challenge Library
Original easy, medium, and hard sessions hide technique tags during the active attempt, keep timers optional, and end with a debrief covering grain, edge cases, and supported strategy choices.
Medium
26 original prompts with hidden variants and debrief notes.
Hard
12 original prompts with hidden variants and debrief notes.
Active interview session
Opening a challenge enters the same workspace with confidence prompt, hidden variants, timer-neutral attempts, and no technique labels before checking.
Debrief
After checking, feedback points to result evidence, Query X-Ray, Mistake Notebook categories, and transfer review rather than a score spectacle.
CH-E01 - stable top products under tied prices
D4CH-E01 - stable top products under tied prices. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-E02 - customers joining in a half-open date range
D4CH-E02 - customers joining in a half-open date range. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-E03 - normalize and match messy city values
D4CH-E03 - normalize and match messy city values. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-E04 - count known versus all return dates
D4CH-E04 - count known versus all return dates. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-E05 - conditional status labels with exact boundaries
D4CH-E05 - conditional status labels with exact boundaries. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-E06 - grouped order counts by channel
D4CH-E06 - grouped order counts by channel. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-E07 - groups above a threshold using HAVING
D4CH-E07 - groups above a threshold using HAVING. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-E08 - customers with at least one completed order
D4CH-E08 - customers with at least one completed order. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-E09 - products with no sales
D5CH-E09 - products with no sales. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-E10 - all teams including zero-ticket teams
D5CH-E10 - all teams including zero-ticket teams. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-E11 - line revenue using captured price
D4CH-E11 - line revenue using captured price. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-E12 - two compatible event streams with source labels
D4CH-E12 - two compatible event streams with source labels. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-E13 - monthly event counts including empty months
D4CH-E13 - monthly event counts including empty months. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-E14 - deterministic first event per account
D4CH-E14 - deterministic first event per account. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-E15 - prior order timestamp per customer
D4CH-E15 - prior order timestamp per customer. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-E16 - insert one row and return generated/default fields
D4CH-E16 - insert one row and return generated/default fields. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-E17 - targeted update with untouched-row assertions
D4CH-E17 - targeted update with untouched-row assertions. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-E18 - interpret a simple scan/sort plan
D4CH-E18 - interpret a simple scan/sort plan. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M01 - customer revenue with zero-order customers
D5CH-M01 - customer revenue with zero-order customers. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M02 - completed and cancelled rates by country
D5CH-M02 - completed and cancelled rates by country. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M03 - top three products per category with ties policy
D5CH-M03 - top three products per category with ties policy. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M04 - latest qualifying order above customer average
D5CH-M04 - latest qualifying order above customer average. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M05 - customers active in both of two periods
D5CH-M05 - customers active in both of two periods. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M06 - customers active in first period but not second
D5CH-M06 - customers active in first period but not second. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M07 - category contribution to monthly revenue
D5CH-M07 - category contribution to monthly revenue. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M08 - monthly revenue and month-over-month change
D5CH-M08 - monthly revenue and month-over-month change. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M09 - three-month rolling average over a complete calendar
D5CH-M09 - three-month rolling average over a complete calendar. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M10 - first-response time from event history
D5CH-M10 - first-response time from event history. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M11 - reopened tickets after a prior resolution
D5CH-M11 - reopened tickets after a prior resolution. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M12 - agent ranking with minimum volume
D5CH-M12 - agent ranking with minimum volume. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M13 - parcels whose latest scan conflicts with shipment status
D5CH-M13 - parcels whose latest scan conflicts with shipment status. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M14 - first missed SLA event per shipment
D5CH-M14 - first missed SLA event per shipment. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M15 - referral self join with lifetime order count
D5CH-M15 - referral self join with lifetime order count. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M16 - current employees and manager chain one level deep
D5CH-M16 - current employees and manager chain one level deep. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M17 - per-order item, payment, and refund totals without fan-out
D5CH-M17 - per-order item, payment, and refund totals without fan-out. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M18 - cohort size and month-one retention
D5CH-M18 - cohort size and month-one retention. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M19 - active subscription MRR at month end
D5CH-M19 - active subscription MRR at month end. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M20 - account movement classification with LAG
D5CH-M20 - account movement classification with LAG. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M21 - safe upsert preserving non-imported fields
D5CH-M21 - safe upsert preserving non-imported fields. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M22 - repair duplicates before adding uniqueness
D5CH-M22 - repair duplicates before adding uniqueness. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M23 - transactionally move child rows and delete parent
D5CH-M23 - transactionally move child rows and delete parent. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M24 - validate a claimed one-row-per-order report
D5CH-M24 - validate a claimed one-row-per-order report. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M25 - compare EXISTS and join approaches with explanation rubric
D5CH-M25 - compare EXISTS and join approaches with explanation rubric. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-M26 - propose an index from predicate/selectivity evidence
D5CH-M26 - propose an index from predicate/selectivity evidence. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-H01 - customer net revenue across independently repeating facts
D5CH-H01 - customer net revenue across independently repeating facts. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-H02 - monthly category rank, share, prior rank, and rank change
D5CH-H02 - monthly category rank, share, prior rank, and rank change. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-H03 - complete cohort-retention matrix with explicit denominator policy
D5CH-H03 - complete cohort-retention matrix with explicit denominator policy. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-H04 - support SLA report with reopen and unresolved semantics
D5CH-H04 - support SLA report with reopen and unresolved semantics. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-H05 - parcel journey reconstruction with tied event timestamps
D5CH-H05 - parcel journey reconstruction with tied event timestamps. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-H06 - subscription new/expansion/contraction/churn MRR bridge
D5CH-H06 - subscription new/expansion/contraction/churn MRR bridge. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-H07 - detect and repair a fan-out executive dashboard query
D5CH-H07 - detect and repair a fan-out executive dashboard query. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-H08 - recursive org rollup with cycle defense
D5CH-H08 - recursive org rollup with cycle defense. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-H09 - idempotent import transaction with validation failures
D5CH-H09 - idempotent import transaction with validation failures. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-H10 - schema repair plus constraint and catalog assertions
D5CH-H10 - schema repair plus constraint and catalog assertions. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-H11 - rolling period metric with missing months and zero prior values
D5CH-H11 - rolling period metric with missing months and zero prior values. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
CH-H12 - correctness-preserving query-quality and plan audit
D5CH-H12 - correctness-preserving query-quality and plan audit. Detect independent one-to-many fan-out and pre-aggregate to a common grain.