SQL LEAD

What is the SQL LEAD function?

LEAD returns data from a subsequent row in the result set, making look-ahead comparisons easy within the same query.

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Compatible dialects for SQL LEAD: PostgreSQL, MySQL 8+, SQL Server, Oracle, Snowflake, BigQuery, Redshift, ClickHouse, SQLite 3.25+, Vertica

SQL LEAD Full Explanation

LEAD is a window (analytic) function that lets you access a value from the next row or any number of rows ahead of the current one without using a self-join. It is evaluated after the FROM, WHERE, GROUP BY, and HAVING clauses but before the final ORDER BY, so it can reference aggregated or calculated columns. LEAD requires an OVER clause that defines an ordering; without a deterministic ORDER BY the result is implementation-dependent. You can specify how many rows ahead to peek (offset) and what default to return when the lead position is outside the partition. If offset is NULL, negative, or zero most engines raise an error. LEAD cannot be used in WHERE or GROUP BY, only in SELECT, ORDER BY, or HAVING. Because it executes as a window function, it preserves the number of rows in the original result, making it ideal for time-series, cohort, and sessionized analyses. Performance is generally O(n) over the partition but can degrade if the ORDER BY requires large sorts or if partitions are extremely wide.

SQL LEAD Syntax

LEAD(value_expression [, offset [, default ]])
  OVER (
        [PARTITION BY partition_expression, ...]
        ORDER BY sort_expression [ASC|DESC] [, ...]
       )

SQL LEAD Parameters

value_expression (any) - Column or expression whose future value you want.
offset (integer) - Number of rows ahead to look (default 1).
default (any) - Value to return when the lead row does not exist (default NULL).

Example Queries Using SQL LEAD

-- 1. Basic usage
SELECT order_id,
       order_date,
       LEAD(order_date) OVER (ORDER BY order_date) AS next_order_date
FROM   orders;

-- 2. Custom offset and default
SELECT user_id,
       event_date,
       LEAD(event_date, 3, '9999-12-31') OVER (PARTITION BY user_id ORDER BY event_date) AS event_date_plus_3
FROM   user_events;

-- 3. Calculating time to next purchase
SELECT customer_id,
       purchase_date,
       LEAD(purchase_date) OVER (PARTITION BY customer_id ORDER BY purchase_date)   AS next_purchase,
       DATE_DIFF('day', purchase_date,
                 LEAD(purchase_date) OVER (PARTITION BY customer_id ORDER BY purchase_date)) AS days_between
FROM   purchases;

Expected Output Using SQL LEAD

Each query returns all original rows plus an extra column that contains the value from the specified future row
If a row does not have a lead row (for example the last row in its partition), the function returns NULL or the supplied default

Use Cases with SQL LEAD

Compute time or value difference to the next event
Flag churn by checking if the next event is missing within a threshold
Determine session boundaries in clickstream data
Create cohort funnels by aligning current and next stage events
Compare current price with the next day price to detect increases

Common Mistakes with SQL LEAD

Omitting ORDER BY in the OVER clause, leading to non-deterministic results
Using LEAD in WHERE or GROUP BY (not allowed)
Supplying an offset of 0 or a negative number
Forgetting to PARTITION BY when analyzing multiple entities, causing cross-entity leakage
Expecting LEAD to filter rows; it only adds columns

First Introduced In

SQL:1999 (window functions); widely adopted in major databases between 2012-2019

Frequently Asked Questions

What is the difference between LEAD and LAG?

LEAD looks forward to future rows, while LAG retrieves values from previous rows. Syntax and parameters are otherwise identical.

Why do I need an ORDER BY in the OVER clause?

Without ORDER BY the database cannot know which row is "next". Some engines allow the omission but produce non-deterministic results.

How do I replace NULL results from LEAD?

Supply the third argument (default). For example: LEAD(sale_amount, 1, 0) returns 0 when no future row exists.

Does LEAD affect query performance?

LEAD itself is O(n), but the ORDER BY inside the window may require sorting. Indexing the sort columns can reduce cost.