Data is requested by an XML application conforming to the XML data model. The data is stored in an SQL database conforming to the relational data model. A mapping component transforms the XML query into an SQL query using a mapping component comprising instructions obtained from an instruction set defined by a mapping algebra. The transformed operation is used to retrieve the data. Once retrieved, the algebraically defined mapping component transforms the SQL data from data conforming to the relational data model to data conforming to the XML data for use in the XML application.