The Aggregator transformation is different from the Expression transformation because you can configure the Aggregator transformation to perform calculations on groups of data. An Expression transformation returns a result on a row by row basis.

For example, you can calculate the average salary for employees in each department of a organization. Configure a group by department number. Configure an expression to calculate the average salary and to return the result for each unique department number.

Use the transformation language to create aggregate expressions.

The Data Integration Service performs aggregate calculations as it reads data and stores the data in an aggregate cache. You can sort the input data to increase performance. The Data Integration Service does not create the cache if you sort the input data.