Calculate time to first action per user

Last updated: September 11, 2025

Quick Overview

Write a query to compute time to first action grouped by date, handling edge cases like nulls and duplicates.

Visa

Data Manipulation (SQL/Python)

Data Scientist

Visa

September 11, 2025

Data Scientist

Phone Screen

Data Manipulation (SQL/Python)

Medium

4,027 solved

Write a query to compute time to first action grouped by date, handling edge cases like nulls and duplicates.

Data manipulation questions at Visa test your ability to work with real-world datasets. This Phone Screen 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

JOIN types and when to use each

Index optimization and query performance

Subqueries and correlated subqueries

NULL handling and COALESCE

Aggregate functions and GROUP BY

Data cleaning and transformation

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?
How would you optimize this query for a table with 100 million rows?
How would you validate the correctness of your query results?
What would you do if this query needs to run every 5 minutes?

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Practice SQL Problems

Sample Answer

Problem Understanding

The task is to compute the time taken for each user to perform their first action, grouped by date. The relevant data likely involves a table (let's call it user_actions) with at least the following...

Approach

Identify the first action timestamp for each user per date. This requires a subquery or a Common Table Expression (CTE) to group by user_id and the date extracted from action_timestamp.
2...

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