Aggregating data by a relative time period
As the cross tab examples in Figure 21-1 show, the capability to aggregate data by relative time periods is useful for comparing data across different time periods. In addition to viewing data by the typical time periods, such as quarter-to-date or previous quarter, you can define time periods, for example, to compare the week before Christmas with the week after Christmas, or the last 15 days in a quarter with the same period last year.
BIRT provides this functionality at the cross tab level where you add a relative time period element and select the time period (Year To Date, Previous Year To Date, and so on) for which to view data. In many cases, you do not need to make any changes to the cube, assuming that the cube already contains at least one time dimension, which is typical. A cube that provides sales data, for example, typically defines time dimensions, such as orderDate or shipDate.

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