The energence and wide deployment of XML technologies in parallel to the (object-) relational paradigm has led to the necessity of managing distributed data spread across XML and object-relational data sources. Our investigation focuses on the design and development of an integration middleware between the application and the data sources, allowing unified access to the entire information stored in autonomous (object-) relation and XML data sources through the corresponding unified global views, an SQL-View,via both query languages, SQL and XQuery. This paper introduces the challenges of the model unification process specially designed for integrated processing of data stored under XML and (object-) relational data models. We prove feasibility and correctness of the proposed approach by examining properties of the constructed unified model and the corresponding unified schemas during all phases of the integration process. Further, we investigate to which extent the schema transformation steps may be conducted automatically during the overall integration process.