The Oracle PL/SQL LAG function is a powerful tool for querying and analyzing data in a database. It allows you to retrieve the value of a column from a previous row in the result set, which can be extremely useful when working with time-series data or when you need to calculate running totals or moving averages.
Syntax
The basic syntax for the PL/SQL LAG function is as follows:
LAG (expression [, offset [, default]]) OVER (ORDER BY sort_expression [ASC | DESC])
expression: The column or expression whose previous value you want to retrieve.
offset: Optional parameter that specifies the number of rows to look back. The default is 1.
default: Optional parameter that specifies the value to return if there is no previous row. The default is NULL.
OVER (ORDER BY column) is an optional clause that specifies the order in which the rows should be processed. If omitted, the function will use the default order of the result set.
sort_expression: The column or expression used to sort the rows.
Example
For example, the following query uses the LAG function to retrieve the previous year’s sales for each salesperson:
SELECT salesperson_id, sales_year, sales_amount,
LAG(sales_amount) OVER (PARTITION BY salesperson_id ORDER BY sales_year) AS prev_year_sales
FROM sales_data;
Conclusion
The Oracle PL/SQL LAG function returns the value of the previous row in a specified column. It allows you to compare values across multiple rows and perform calculations based on the previous row’s values.
Overall, the LAG function is a valuable tool for data analysis in Oracle databases. By allowing you to retrieve values from previous rows in a result set, it can help you gain insights into trends and patterns that might be difficult to spot otherwise.