Write a query to find conversion rate from impressions
Last updated: July 15, 2025
Quick Overview
Write a SQL query to calculate conversion rate from the impressions table, considering nulls and duplicates.
Slack
July 15, 2025119
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2,151 solved
Write a SQL query to calculate conversion rate from the impressions table, considering nulls and duplicates.
Data manipulation questions at Slack test your ability to work with real-world datasets. This Take-home Project question evaluates your SQL proficiency, understanding of data modeling, and ability to derive insights from raw data.
What the Interviewer Expects
- Use advanced SQL features: window functions, CTEs, subqueries
- Write efficient queries that avoid common performance pitfalls
- Handle complex data transformations with multiple joins and aggregations
- Discuss indexing strategy and query optimization
- Address data quality issues: duplicates, missing values, outliers
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 validate the correctness of your query results?
- What indexes would you create to support this query?
- What would you do if this query needs to run every 5 minutes?
- How would you handle slowly changing dimensions in this scenario?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task is to calculate the conversion rate from a table called impressions. This table likely contains records of user interactions, including fields such as user_id, impression_id, and possib...
Approach
- Identify Unique Impressions: Use a DISTINCT clause to ensure we count each impression only once.
- Count Conversions: Use a WHERE clause to filter impressions that resulted in a conversi...
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