Technical Skills

SQL and Data Querying Test

Assess SQL knowledge across joins, filtering, aggregation, database structure and data querying.

45 minMedium15 questions15 sections
SQL joinsAggregationsFilteringDatabase structure
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Topics covered

Joins and AggregationLatest RecordRunning TotalsCohort AnalysisIndexingExecution PlansTransactionsUpsertsNULL SemanticsDeduplicationPaginationIsolation LevelsSchema EvolutionResult ValidationQuery Review

Sample questions

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Joins and AggregationMid-level

Schema: customers(id, name) orders(id, customer_id, total, status) order_items(id, order_id, product_id, quantity) A report must return every customer, including customers with no orders, and their paid_order_revenue. A first attempt joins all three tables and produces inflated revenue. Write the query and explain why the first attempt duplicates totals.

Latest RecordMid-level

Schema: subscription_events(customer_id, event_time, status, event_id) Return the latest status for each customer. Events can share the same event_time. Write a deterministic query and explain the tie-breaker.

Running TotalsMid-level

Schema: transactions(account_id, occurred_at, transaction_id, amount) Return every transaction with the account's running balance ordered by time. Multiple transactions may share a timestamp.

Cohort AnalysisMid-level

Design a SQL approach for monthly signup cohorts and month-0 to month-6 active-user retention. Define the cohort, activity event, denominator and treatment of users with insufficient observation time.

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