The Rank transformation selects the top or bottom range of data. Use the Rank transformation to return the largest or smallest numeric values in a group. You can also use the Rank transformation to return strings at the top or bottom of the mapping sort order.
For example, you can use a Rank transformation to select the top 10 customers by region. Or, you might identify the three departments with the lowest expenses in salaries and overhead.
The Rank transformation differs from the transformation functions MAX and MIN because the Rank transformation returns a group of values, not just one value. While the SQL language provides many functions designed to handle groups of data, identifying top or bottom strata within a set of rows is not possible using standard SQL functions.
The Rank transformation is an active transformation because it can change the number of rows that pass through it. For example, you configure the transformation to select the top 10 rows from a source that contains 100 rows. In this case, 100 rows pass into the transformation but only 10 rows pass from the Rank transformation to the downstream transformation or target.
When you run a mapping that contains a Rank transformation,
caches input data until it can perform the rank calculations.