What is the purpose of the Oracle lag function?

The LAG function in Oracle is used to access the data from the previous row in the same query result set. It can be used in a SELECT statement to retrieve the value of a specific column from the previous row and add it as a new column in the current row. This can be helpful for calculating differences between rows or performing similar comparison operations. The syntax of the LAG function is as follows:

LAG (column, offset [, default]) OVER ( [query_partition_clause] order_by_clause )

Specify the column to be accessed, the offset indicating the number of rows to access from the beginning, and the optional default parameter specifying the default value to return when there is no previous row. The query_partition_clause determines the partition rule, and the order_by_clause determines the sorting rule.

In general, the purpose of the LAG function is to retrieve data from the previous row within the same result set, in order to compare or calculate in the current row.