Specifies how the nodes returned from an XPath expression in the node attribute should be aggregated to obtain a single value for the document.
The aggregation attribute is an optional attribute of the field element.
The aggregation attribute can either be an aggregation action or an indication of the particular element in the collection. It uses the following aggregation actions: sum, count, average, min, max, first, last, merge, and plaintext.
The following is an example of the aggregation attribute as it is used in the field element: