Write a query to find rolling 7-day average from transactions
Last updated: February 2, 2026
Quick Overview
Write a SQL query to calculate rolling 7-day average from the transactions table, considering nulls and duplicates.
Instacart
February 2, 20262
6
2,558 solved
Write a SQL query to calculate rolling 7-day average from the transactions table, considering nulls and duplicates.
This question from Instacart's Phone 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 optimize this query for a table with 100 million rows?
- Can you rewrite this without using subqueries?
- What indexes would you create to support this query?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task is to calculate the rolling 7-day average of transaction amounts from the transactions table. The relevant data includes the transaction date and the amount for each transaction. We need to...
Approach
- Data Selection: Start by selecting the date and amount columns from the
transactionstable. - NULL Handling: Use
COALESCEto replace NULL transaction amounts with 0, as they should not...
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