SQL ROW_NUMBER

Understanding ROW_NUMBER

ROW_NUMBER is a window function that assigns a unique sequential number to each row within a partition of a result set. Unlike RANK and DENSE_RANK which handle ties by assigning the same number, ROW_NUMBER always assigns unique numbers even when values are equal, making it perfect for pagination, finding nth occurrences, or eliminating duplicates.

Basic Syntax

Common ROW_NUMBER Interview Questions

What's the difference between ROW_NUMBER, RANK, and DENSE_RANK?
How would you find the most recent video for each user?
How can you implement pagination using ROW_NUMBER?
How would you eliminate duplicate rows in a table?

Common ROW_NUMBER Patterns

1. Finding Latest Records

Get the most recent video upload for each user.

2. Implementing Pagination

Create paginated results for a video listing.

3. Eliminating Duplicates

Remove duplicate interactions while keeping the most recent one.

Advanced ROW_NUMBER Examples

Example 1: User Engagement Analysis

Analyze the first and last interaction for each user:

Example 2: Top N Videos Per User

Find the top 3 most viewed videos for each user:

Best Practices for ROW_NUMBER

1. Handle Ties Appropriately

When dealing with ties:

ROW_NUMBER will arbitrarily assign different numbers to tied values
Add additional ORDER BY columns to break ties consistently
Consider using RANK or DENSE_RANK if ties should share the same number

2. Optimize Performance

Keep your ROW_NUMBER queries efficient:

Index columns used in PARTITION BY and ORDER BY
Filter data before applying ROW_NUMBER when possible
Use appropriate partition sizes to avoid memory issues

3. Common Use Cases

Best scenarios for using ROW_NUMBER:

Implementing pagination
Finding first/last records per group
Eliminating duplicates
Creating sequential IDs within groups

Interview Master

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