Write a query to find conversion rate from events
Last updated: April 21, 2026
Quick Overview
Write a SQL query to calculate conversion rate from the events table, considering nulls and duplicates.
Mastercard
April 21, 2026590
7
3,415 solved
Write a SQL query to calculate conversion rate from the events table, considering nulls and duplicates.
This question from Mastercard's Technical Screen tests practical data skills. The interviewer wants to see clean, efficient queries that handle edge cases like NULLs, duplicates, and large datasets.
What the Interviewer Expects
- Solve complex analytical problems with elegant, readable SQL
- Optimize queries for large-scale datasets with partitioning and indexing
- Use recursive CTEs, lateral joins, and advanced window functions
- Design the data model alongside the query solution
- Discuss trade-offs between SQL and programmatic approaches (Python/pandas)
- Consider the operational aspects: query scheduling, incremental processing
Key Topics to Cover
How to Approach This
- Clarify the schema and expected output format before writing queries.
- Use CTEs (WITH clauses) to break complex queries into readable steps.
- Consider window functions (ROW_NUMBER, RANK, LAG, LEAD) for ranking and sequential analysis.
- Watch for NULLs, duplicates, and edge cases in JOINs and GROUP BY.
- For pandas, prefer vectorized operations over row-by-row iteration.
Possible Follow-up Questions
- How would you handle this if the data was spread across multiple databases?
- What indexes would you create to support this query?
- How would you handle slowly changing dimensions in this scenario?
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Practice SQL ProblemsSample Answer
Problem Understanding
In this problem, we are tasked with calculating the conversion rate from an events table. This table likely contains various user events, including actions such as clicks, sign-ups, and purchases. T...
Approach
- Identify the events: Determine which events represent a conversion (e.g., 'purchase') and which represent initial interactions (e.g., 'click' or 'signup').
- Data Cleaning: Use window func...
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