Abstract: In a data processing system, hierarchical documents or hierarchical messages are mapped to a Nested Relational Data Model to allow for transformation and manipulation using declarative statements. The resulting nested data can be converted to a relational format and mapped to multiple relational tables, and/or converted from a nested relational format to an external hierarchical format, such as XML. The system can specify and execute declarative rules to extract, transform, integrate, load and update hierarchical and relational data. The system can also be used for extending documents with relational and non-relational data and applying updates based on these documents to relational database targets. The system can also be used for mapping Nested Relational Data to function calls that accept tables as parameters and return multiple scalar and table parameters as output.