LEAD Function

Overview

LEAD() accesses data from a succeeding row at a given offset within a partition, without needing a SELF JOIN.

Learning Objectives

• Retrieve values from future rows using LEAD(column, offset, default).
• Calculate time gaps until the next event occurs.
• Conduct sessionization and next-event analysis.

Detailed Concept Explanation

LEAD() looks forward $N$ rows ahead of the current row within the window partition: LEAD(column, offset, default_value)

Code Examples

SQL

-- Calculate time elapsed until a user's NEXT click event
SELECT 
    user_id,
    event_time AS current_click,
    LEAD(event_time, 1) OVER (PARTITION BY user_id ORDER BY event_time) AS next_click,
    DATEDIFF('minute', event_time, LEAD(event_time, 1) OVER (PARTITION BY user_id ORDER BY event_time)) AS mins_to_next_click
FROM user_events;

Best Practices

• Specify Partitioning: Always include PARTITION BY when analyzing user-level events so LEAD() does not bleed into the next user's records.

Interview Perspective

Interactive Challenges

Summary

LEAD() looks ahead to future rows in a partition, useful for calculating time gaps until next events.