Window function: lead/lag over date

Last updated: March 19, 2026

Quick Overview

Use window functions to compute running total partitioned by date.

Snowflake

Data Manipulation (SQL/Python)

Data Scientist

Snowflake

March 19, 2026

Data Scientist

Technical Screen

Data Manipulation (SQL/Python)

Medium

2,792 solved

Use window functions to compute running total partitioned by date.

This question from Snowflake's Technical 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

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

Data cleaning and transformation

Index optimization and query performance

JOIN types and when to use each

Aggregate functions and GROUP BY

Window functions (ROW_NUMBER, RANK, LAG, LEAD)

Date/time manipulation

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 handle slowly changing dimensions in this scenario?
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 validate the correctness of your query results?

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Practice SQL Problems

Sample Answer

Problem Understanding

The goal is to compute a running total of a specific metric (e.g., sales, revenue) partitioned by date using window functions in SQL. We will assume we have a table named sales_data with columns inc...

Approach

Start by selecting the necessary fields from the sales_data table, including sale_date and amount.
Use the SUM() window function to calculate the running total of amount over the parti...

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