👀 SQL LEAD and LAG Functions: Access Previous and Next Row Values in SQL

Learn how to use SQL LEAD and LAG functions to access previous and next row values without self-joins. These powerful window functions make it easy to compare records, calculate changes, and analyze trends across time or transactions.

In this tutorial, you’ll explore practical SQL LEAD LAG examples and common interview questions to master how these functions look ahead and behind within your data.

🧐 What Are SQL LEAD and LAG Functions?

In simple terms:

• LEAD() lets you grab the next row's value
• LAG() grabs the previous one

They work inside a window, meaning they operate over a group of rows — without collapsing them like GROUP BY.

Imagine this:

If we write:

SELECT OrderID, Amount,
       LAG(Amount) OVER (PARTITION BY CustomerID ORDER BY OrderDate) AS PrevAmount
FROM my_table;

We’ll get:

Magical? Close. It’s just window functions being awesome.

🔍 When Should You Use LEAD() or LAG()?

Use these functions any time you want to:

• Compare rows in sequence
• Calculate differences or changes over time
• Find trends like “what changed since last week?”
• Predict next values, e.g. upcoming appointments, stockouts, or renewals

💼 SQL LEAD and LAG Interview Questions

Many SQL interview questions test whether you recognize when to use LEAD or LAG. These functions appear in problems asking you to compare previous and next row values or detect changes between records. If you spot patterns like these, it’s probably a LEAD LAG interview question in SQL:

• “Show each order and the previous order amount for that customer.”
• “Compare each employee’s current and prior salary.”
• “Find how today’s sales differ from the next day’s sales.”
• “Identify rows where a value increased or decreased compared to last time.”

These questions test your ability to think in sequences, recognize PARTITION BY logic, and understand how LAG() and LEAD() look backward and forward within a dataset.

🧱 Core SQL Syntax for LEAD() and LAG()

-- Get previous order amount
LAG(TotalAmount) OVER (
  PARTITION BY CustomerID
  ORDER BY OrderDate
) AS PrevAmount

-- Get next delivery date
LEAD(DeliveryDate) OVER (
  PARTITION BY ProductID
  ORDER BY OrderDate
) AS NextDelivery

⏩ Looking Further Ahead (or Behind): Using LEAD/LAG with Offsets

By default, LAG() and LEAD() return the value just one row away. But you can also “peek” two or more rows forward or backward by adding an offset argument.

SELECT CustomerID, OrderDate, Amount,
       LAG(Amount, 2) OVER (PARTITION BY CustomerID ORDER BY OrderDate) AS TwoOrdersAgo
FROM Orders;

This retrieves the value from two rows earlier within each customer's order history. Similarly, LEAD(Amount, 3) would look three rows ahead instead of one.

Offsets are especially helpful when analyzing trends, such as comparing a customer's spending to two orders ago or spotting repeating patterns in sensor data.

🚯 Handling NULL Values in LEAD and LAG Results

By default, LAG() and LEAD() return NULL when there’s no previous or next row. But what if you want to fill that in with something useful — like 0, 'N/A', or "no change"?

You’ve got two main options:

✅ Option 1: Built-in Default Value (3rd Argument)

SQL lets you set a default value right inside the function:

LAG(Salary, 1, 0)

This means:

• Look 1 row back
• If no row exists (e.g., first row), return 0 instead of NULL

Same applies to LEAD():

LEAD(DeliveryDate, 1, '2999-12-31')

✅ Option 2: Wrap with COALESCE()

This gives you even more flexibility — especially when chaining or nesting:

COALESCE(LAG(Amount) OVER (...), 0)

Or if you're doing something dynamic like this:

COALESCE(
  LAG(Salary) OVER (PARTITION BY Department ORDER BY HireDate),
  Salary -- fallback to current salary if none before
)

✨ LEAD vs LAG: Which Function Should You Use?

Both are correct and efficient. Pick based on readability and use case.

🧪 Gotchas with Ties, Missing Rows, and Data Gaps

• Ties in ORDER BY? SQL doesn't break them unless you do. Add a tiebreaker column if needed.
• Dates with gaps? LAG won’t fill them in — it just looks at row position, not calendar gaps.

⚠️ Common Pitfalls When Using LEAD and LAG

Even Xorthax got tripped up once or twice. Here are mistakes to dodge:

❌ Why LEAD and LAG Don’t Support Frame Clauses

Unlike SUM() or AVG(), these functions don’t use ROWS BETWEEN. Why? Because they just fetch a specific row ahead or behind — not a range.

🔗 Using LEAD/LAG Inside a CTE (Common Table Expression)

Sometimes, you’ll want to calculate with LAG/LEAD, then filter on it. That’s where CTEs come in handy.

WITH Ordered AS (
  SELECT
    CustomerID,
    OrderID,
    TotalAmount,
    LAG(TotalAmount) OVER (PARTITION BY CustomerID ORDER BY OrderDate) AS PrevAmount
  FROM Orders
)
SELECT *
FROM Ordered
WHERE TotalAmount < PrevAmount;

This shows all orders where the customer spent less than the time before.

🌟 Combining LEAD, LAG, and Other SQL Window Functions

You’re not limited to one! This is totally valid:

SELECT
  OrderID,
  TotalAmount,
  LAG(TotalAmount) OVER (...) AS Prev,
  LEAD(TotalAmount) OVER (...) AS Next

You can also combine with ROW_NUMBER() to get order positions or track gaps.

🤔 LEAD/LAG vs Aggregate Functions — What’s the Difference?

Let’s be clear:

🛠 Debugging LEAD/LAG Queries Like a Pro

• Use ROW_NUMBER() temporarily to see your row order.
• Expect NULLs on edge rows — they’re normal!

• Use COALESCE() or the optional argument to fill NULLs if needed:

  COALESCE(LAG(x), 0)
  or 
  LAG(Salary, 1, 0)
  

↻ Related Topics

You might also like: FIRST_VALUE and Related Functions, ROW_NUMBER and Ranking Functions, Trends with SUM + ROWS BETWEEN