Reporting including filling data gaps and handling uncategorized data

A reporting system is described herein that allows a report author to declare data reporting structures that specify to a reporting application how to dynamically categorize data with changing or potentially unknown characteristics. The reporting system may extend RDL and the data grouping provided by Microsoft SQL Server Reporting Services by adding new elements to the XML-based RDL schema. The reporting system allows the report author to specify for the system to fill gaps in the data, so that the report has a similar layout even as data changes from period to period. The reporting system also allows the report author to specify whether data that does not fit any predefined group bucket is displayed in a report. Thus, the reporting system allows unsophisticated database users to define reports that group data consistently regardless of missing values or other changes in the underlying data.