Write a query to find conversion rate from users
Last updated: August 10, 2025
Quick Overview
Write a SQL query to calculate conversion rate from the users table, considering nulls and duplicates.
Instacart
August 10, 20255
6
2,365 solved
Write a SQL query to calculate conversion rate from the users table, considering nulls and duplicates.
Instacart asks this during the Phone Screen 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
- 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
- What would you do if this query needs to run every 5 minutes?
- What indexes would you create to support this query?
- How would you handle slowly changing dimensions in this scenario?
- How would you handle this if the data was spread across multiple databases?
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Practice SQL ProblemsSample Answer
Problem Understanding
To calculate the conversion rate from the users table, we need to identify two key metrics: the total number of users and the total number of users who converted (i.e., made a purchase). The conversio...
Approach
- Identify the relevant columns: We'll focus on user IDs (to determine unique users) and a conversion indicator (e.g., a purchase flag).
- Handle duplicates: Use a CTE with the ROW_NUMBER(...
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