Write a query to find cumulative sum from impressions
Last updated: January 28, 2026
Quick Overview
Write a SQL query to calculate cumulative sum from the impressions table, considering nulls and duplicates.
TikTok
January 28, 202642
13
2,155 solved
Write a SQL query to calculate cumulative sum from the impressions table, considering nulls and duplicates.
Data manipulation questions at TikTok test your ability to work with real-world datasets. This Onsite 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
- Can you rewrite this without using subqueries?
- What indexes would you create to support this query?
- How would you handle slowly changing dimensions in this scenario?
- How would you optimize this query for a table with 100 million rows?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task requires calculating the cumulative sum of the 'impressions' column from an 'impressions' table. The key points to consider include:
- The data may contain NULL values, which should be treate...
Approach
To build the SQL query, the following steps will be followed:
- Data Cleaning: Use COALESCE to replace NULL values in the 'impressions' column with zero.
- Deduplication: If duplicates exist...
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