SQL LEAD Function

Understanding the LEAD Function

The LEAD function is a window function that provides access to a row at a specified physical offset that comes after the current row in the result set. This makes it perfect for comparing current values with future values, analyzing upcoming trends, and calculating forward-looking changes.

Basic Syntax

Common LEAD Interview Questions

How would you compare each video's views with the next uploaded video?
How can you identify videos that have fewer views than their next upload?
What's the difference between LEAD and LAG functions?
How would you handle the last row where there is no next value?

Common LEAD Patterns

1. Basic Usage

Compare current row with the next row.

2. Partitioned Comparisons

Compare values within specific groups.

3. Custom Offset and Default Values

Look ahead multiple rows and handle NULL values.

4. Future Performance Analysis

Analyze upcoming trends and patterns.

Best Practices for LEAD

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 LEAD 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 LEAD queries efficient:

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

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