SQL LAG Function

Understanding the LAG Function

The LAG function is a window function that provides access to a row at a specified physical offset that comes before the current row in the result set. This makes it perfect for comparing current values with previous values, analyzing trends, and calculating period-over-period changes.

Basic Syntax

Common LAG Interview Questions

How would you calculate the view count change between consecutive video uploads?
How can you find videos that have more views than their previously uploaded video?
What's the difference between LAG and LEAD functions?
How would you handle the first row where there is no previous value?

Common LAG Patterns

1. Basic Usage

Compare current row with the previous row.

2. Partitioned Comparisons

Compare values within specific groups.

3. Custom Offset and Default Values

Look back multiple rows and handle NULL values.

4. Trend Analysis

Analyze patterns and trends in sequential data.

Best Practices for LAG

1. Handle NULL Values

Consider these strategies for NULL handling:

Use the default_value parameter to provide fallback values
Filter out NULL results if they're not meaningful
Use COALESCE with LAG for custom NULL handling

2. Ordering Matters

Pay attention to your ORDER BY clause:

Ensure deterministic ordering with unique columns
Consider adding secondary sort columns for ties
Use appropriate date/time types for temporal ordering

3. Performance Optimization

Keep your LAG queries efficient:

Index columns used in ORDER BY and PARTITION BY
Filter data before applying LAG
Consider materialized views for frequent calculations

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