Introduction
SQL Server provides several useful functions to manipulate data within a table. The LEAD and LAG functions are two such functions that allow you to access data from other rows in the same result set. These functions can be very helpful in analyzing data and creating complex queries.
What is the LEAD function?
The LEAD function is used to access data from the next row in a result set. The syntax of the LEAD function is as follows.
LEAD(expression, offset, default) OVER (ORDER BY order_expression)
The expression parameter is the column you want to access from the next row, the offset parameter specifies the number of rows ahead you want to access, and the default parameter is the value to return if there is no next row. The ORDER BY clause specifies the order of the result set.
For example, let’s say you have a table called sales with the following data.
date | amount
-----------|-------
2021-01-01 | 100
2021-01-02 | 200
2021-01-03 | 150
2021-01-04 | 300
You can use the LEAD function to access the amount from the next row.
SELECT date, amount, LEAD(amount, 1, 0) OVER (ORDER BY date) as next_amount
FROM sales
The result of this query will be.
date | amount | next_amount
-----------|--------|------------
2021-01-01 | 100 | 200
2021-01-02 | 200 | 150
2021-01-03 | 150 | 300
2021-01-04 | 300 | 0
What is the LAG function?
The LAG function is used to access data from the previous row in a result set. The syntax of the LAG function is similar to the LEAD function.
LAG(expression, offset, default) OVER (ORDER BY order_expression)
The expression parameter is the column you want to access from the previous row, the offset parameter specifies the number of rows behind you want to access, and the default parameter is the value to return if there is no previous row. The ORDER BY clause specifies the order of the result set.
For example, let’s say you have a table called expenses with the following data.
date | amount
-----------|-------
2021-01-01 | 100
2021-01-02 | 200
2021-01-03 | 150
2021-01-04 | 300
You can use the LAG function to access the amount from the previous row.
SELECT date, amount, LAG(amount, 1, 0) OVER (ORDER BY date) as previous_amount
FROM expenses
The result of this query will be.
date | amount | previous_amount
-----------|--------|----------------
2021-01-01 | 100 | 0
2021-01-02 | 200 | 100
2021-01-03 | 150 | 200
2021-01-04 | 300 | 150
Conclusion
The LEAD and LAG are useful functions, and can be very helpful in analyzing data and creating complex queries. With the examples provided above, we should have a good understanding of how to use these functions.