Define the mapping output in the

Outputs

tab of the Properties view. Configure a name for the mapping output, a data type for the result, and indicate what type of aggregation to perform to return a result.

After you define the mapping output name and the output type, configure an Expression transformation in the mapping. In the Expression transformation, define the output expression that you want to aggregate. The expression can contain a port name or it can contain an expression with ports, functions, and parameters.