Write a query to find conversion rate from messages
Last updated: March 26, 2026
Quick Overview
Write a SQL query to calculate conversion rate from the messages table, considering nulls and duplicates.
Cockroach Labs
March 26, 2026200
4
3,418 solved
Write a SQL query to calculate conversion rate from the messages table, considering nulls and duplicates.
Cockroach Labs asks this during the Take-home Project because data engineering skills are critical for the role. You should be comfortable with complex joins, window functions, CTEs, and performance optimization.
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
- What indexes would you create to support this query?
- Can you rewrite this without using subqueries?
- How would you handle this if the data was spread across multiple databases?
- How would you handle slowly changing dimensions in this scenario?
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Practice SQL ProblemsSample Answer
Problem Understanding
The problem requires calculating the conversion rate from a messages table. This table likely contains columns such as message_id, user_id, created_at, and status, where status indicates w...
Approach
- Identify the relevant columns in the
messagestable and their meanings. - Filter out any messages with NULL status as they do not contribute to the conversion rate.
- Use aggregate function...
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