Each record in the report is a subreport. Each time we pass a different city as a
parameter, a report for the appropriate city is generated.
The <subreport> JRXML element can contain a number of sub-elements that were
not used in this example. These sub-elements include:
<dataSourceExpression> : This is used to pass a datasource to the
subreport. This datasource is usually obtained from a parameter in the
master report or by using the built-in REPORT_DATA_SOURCE parameter to
pass the parent report's datasource to the subreport.