It is enough to place a text component with an aggregate function in a GroupFooter to output footer by group. Also, the footer of a group may be placed in a GroupHeaderBand. For example, in order to count the number of rows in each group in a Text component the following expression can be used:
{Count()}
A component is placed in the GroupFooter band.
After rendering, it is possible to see that in the footer of each group calculation by number of rows is done.
