5. A computer program product, stored on a digital storage medium, comprising: a data model having a set of entity types and a set of attributes for each entity type of the set of entity types, customizing data for the data model, the customizing data being indicative of data sources for the attributes and being descriptive of data structures being provided by the data sources; and program means for performing the method of: a) receiving a query from an application program, the query requesting attribute data for at least a first attribute of the set of attributes of one of the entity types; b) processing the query by a query processing module that determines whether data sources and data structures are available based on the customizing data in the data model; c) determining from the customizing data, if a single data source for the first attribute data of the one of the entity types is available; d) if a single data source is available, providing a query result from the single data source; e) if a single data source is not available, determining at least first and second ones of the data sources which in combination comprise the first attribute data of the one of the entity types; f) generating a sub-query for each one of the at least first and second data sources, a first sub-query being generated to obtain a first tabular data from the first data source and a second sub-query being generated to obtain a second tabular data from the second data source; and g) combining the results of the sub-queries from the at least first and second data sources to provide a query result, wherein the determination of the at least first and second data sources is made by a data source descriptor, the data source descriptor allowing replacement of a data source without making any modifications to the data source descriptor.
5. A computer program product, stored on a digital storage medium, comprising: a data model having a set of entity types and a set of attributes for each entity type of the set of entity types, customizing data for the data model, the customizing data being indicative of data sources for the attributes and being descriptive of data structures being provided by the data sources; and program means for performing the method of: a) receiving a query from an application program, the query requesting attribute data for at least a first attribute of the set of attributes of one of the entity types; b) processing the query by a query processing module that determines whether data sources and data structures are available based on the customizing data in the data model; c) determining from the customizing data, if a single data source for the first attribute data of the one of the entity types is available; d) if a single data source is available, providing a query result from the single data source; e) if a single data source is not available, determining at least first and second ones of the data sources which in combination comprise the first attribute data of the one of the entity types; f) generating a sub-query for each one of the at least first and second data sources, a first sub-query being generated to obtain a first tabular data from the first data source and a second sub-query being generated to obtain a second tabular data from the second data source; and g) combining the results of the sub-queries from the at least first and second data sources to provide a query result, wherein the determination of the at least first and second data sources is made by a data source descriptor, the data source descriptor allowing replacement of a data source without making any modifications to the data source descriptor. 7. The computer program product of claim 5 , wherein the first data source relates the entity type data of the one of the entity types to second attribute data of a second attribute of the set of attributes, and wherein the second data source relates the second attribute data to the first attribute data.