Window functions calculate a return value for every input row of a table, based on a group of rows.
A window function performs a calculation across a set of table rows that are related to the current row. You can also perform this type of calculation with an aggregate function. But unlike regular aggregate functions, a window function does not group rows into a single output row. The rows retain unique identities.
You can define the LEAD and LAG analytic window functions in an Expression transformation. LEAD and LAG give access to multiple rows within a table, without the need for a self-join.