🧠The SQL Cheat Code That Will Instantly Upgrade Your Interview Skills

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When it comes to SQL interviews, there’s one category of questions that trips up even the most experienced analysts and data engineers:

“Compare each row to the previous or next row.”

Whether it’s tracking changes in customer behavior, detecting duplicates, or identifying time gaps — if you don’t know LEAD() and LAG(), you’re in for a bumpy ride.

But if you do know how to wield them… they’re a superpower.

🚀 What Are LEAD and LAG?

LEAD() and LAG() are SQL window functions that allow you to look forward or backward in your data — without complex joins or subqueries.

They work by returning the value of another row relative to the current one, based on the order you define.

Syntax:

LAG(column_name, offset, default_value) OVER (PARTITION BY ... ORDER BY ...)
LEAD(column_name, offset, default_value) OVER (PARTITION BY ... ORDER BY ...)

🔥 Common Real-World Use Cases

Let’s explore powerful scenarios where LEAD() and LAG() shine — especially ones that frequently appear in interviews and real-life data projects.

1. Detecting Value…