You can apply one or more subtotals in your query definition. In Reporting Services terms, subtotals are “groups”. When you enable Subtotal for a Display enabled field, a group is added. The groups nest in the Order you specify after Subtotal.
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In the output, group headers appear for the first group only; group footers, with a count and the value being grouped, print for all groups.
Using subtotals in the definition of a simple Activity List query