MySQL | LEAD() and LAG() Function

The LEAD() and LAG() functions in MySQL are powerful window functions introduced in version 8.0. They allow users to access data from preceding or following rows without the need for self-joins or subqueries, making them invaluable for analyzing sequential or time-series data.

In this article, we will learn the LEAD() and LAG() functions in MySQL, covering their syntax, parameters, and key differences. You’ll learn how to use these functions with real-world examples, explore their role in window function operations

What are LEAD() and LAG() Functions in MySQL?

The LAG() function is used to get value from the row that precedes the current row.

The LEAD() function is used to get value from a row that succeeds the current row.

Syntax: 

For LEAD() Function- 

LEAD(expr, N, default) 
          OVER (Window_specification | Window_name)

For LAG() Function- 

LAG(expr, N, default) 
          OVER (Window_specification | Window_name)

The N and default argument in the function is optional.

Parameters used:  

1. expr: It can be a column or any built-in function.
2. N: It is a positive value which determine number of rows preceding/succeeding the current row. If it is omitted in query then its default value is 1.
3. default: It is the default value return by function in-case no row precedes/succeedes the current row by N rows. If it is missing then it is by default NULL.
4. OVER(): It defines how rows are partitioned into groups. If OVER() is empty then function compute result using all rows.
5. Window_specification: It consist of query partition clause which determines how the query rows are partitioned and ordered.
6. Window_name: If window is specified elsewhere in the query then it is referenced using this Window_name.

Examples of LEAD() and LAG() Functions

Consider a “contest” table:- 

In the above table, “c_id” represents contest id and “start_date” and “end_date” represents starting and ending date of contest respectively. 

Example 1: Calculate Overlapping Days Between Contests

Problem description: We have to find number of days a contest will collapse with the next contest i.e no. of days on which both contests are held.

Query: 

SELECT c_id, 
       start_date, 
       end_date,
       DATEDIFF(LEAD(start_date) OVER (ORDER BY start_date), end_date) + 1 AS no_of_days
FROM contest;

Explanations:

In the above query “end_date” return ending date of the current contest and lead(start_date) return next contest starting date. So, the difference between these dates plus 1 will return no. of days the contest will collide.

Here, window specification is given by “order by” clause, which represent that lead() function will operate on table order by their “start_date” in increasing order. Since there is no partition clause so, whole table is taken as a single window.

Output:

Since, there is no contest after contest 7 i.e (c_id=7). So, lead(start_date) returns NULL value.

Note:The LEAD() and LAG() function are always used with OVER(). Missing of an over clause will raise an error.

Example 2: Retrieve Preceding Data for Analysis

Query:

SELECT 
    c_id, 
    start_date, 
    end_date, 
    LAG(end_date) OVER (ORDER BY start_date) AS prev_end_date,
    DATEDIFF(start_date, LAG(end_date) OVER (ORDER BY start_date)) AS gap_days
FROM contest;

Output:

Conclusion

The LEAD() and LAG() functions in MySQL are essential for performing sequential data analysis, making it easier to access previous or next rows without complex joins. Whether you’re comparing trends, detecting gaps, or monitoring progressions, these functions simplify query writing and enhance performance. By understanding the syntax, use cases, and real-world applications, you can leverage LEAD() and LAG() effectively in your database operations.