The summarizer uses the groupBy property to determine whether or not a data item matches any of the data items in its list. The groupBy property lists the data item properties that are considered in the matching process. To compare two data items, the summarizer compares each of the values in the groupBy property. If each of the values match, the data items are said to match for this summarizer. The net effect is analogous to the use of the SQL GROUP BY clause used in many relational database reports.