PL/SQL What is the difference between Lead and Lag functions

In Oracle PL/SQL, the Lead and Lag functions are analytical functions used to access data from rows that come before or after the current row within a result set. These functions are particularly useful for performing calculations involving sequential data or time series analysis. Although they serve similar purposes, there are important differences between the Lead and Lag functions.

Lag Function

The Lag function allows you to retrieve data from a previous row within the result set. It takes three arguments: the column or expression to retrieve, the number of rows back to look, and an optional default value if no previous row exists. Here’s the general syntax:

LAG(column/expression, offset, default_value) 
OVER (ORDER BY column/expression)

For example, consider a table with sales data sorted by date. To calculate the difference in sales from the previous day, you can use the Lag function:

SELECT date, sales, 
LAG(sales, 1, 0) OVER (ORDER BY date) AS previous_sales
FROM sales_table;

In this example, the Lag function retrieves the sales value from the previous row, and if there is no previous row, it returns 0 as the default value.

Lead Function

The Lead function allows you to retrieve data from a subsequent row within the result set. It works similarly to the Lag function but retrieves data from rows that follow the current row. The syntax is the same as the Lag function:

LEAD(column/expression, offset, default_value) 
OVER (ORDER BY column/expression)

Continuing with the previous example, let’s calculate the difference in sales with the following day’s sales using the Lead function:

SELECT date, sales, 
LEAD(sales, 1, 0) OVER (ORDER BY date) AS next_day_sales
FROM sales_table;

Here, the Lead function retrieves the sales value from the subsequent row. If there is no subsequent row, it returns 0 as the default value.

What is the difference between Lead and Lag functions

Here are the differences between Lead and Lag functions:

The Lag function looks backward, while the Lead function looks forward within the result set.
The Lag function retrieves data from the previous row, while the Lead function retrieves data from the subsequent row.
Both functions take the same arguments: the column or expression to retrieve, the number of rows to look, and an optional default value.
If the specified offset goes beyond the boundaries of the result set, both functions return the default value.
The ORDER BY clause is essential for both functions to determine the sequence of rows.

In summary, the Lag and Lead functions in Oracle PL/SQL provide a convenient way to access data from preceding or succeeding rows. They are powerful tools for performing calculations or comparisons involving sequential data, such as time series analysis or tracking changes over time.