1. A method comprising: providing an entity-relationship (ER)-extensible markup language (XML) hybrid data model of a relational database comprising XML data; receiving a user setting of a first annotation of a first entity in the ER-XML hybrid data model marking the first entity as not to be transformed as XML data; receiving a user setting of a second annotation of a second entity in the ER-XML hybrid data model marking the second entity as to be transformed as XML data; and transforming, by a computing processor, the ER-XML hybrid data model to a physical data model of the relational database, comprising: based on the first annotation of the first entity marking the first entity as not to be transformed as XML data, transforming the first entity in the ER-XML hybrid data model to a first table in the physical data model; based on the second annotation of the second entity marking the second entity as to be transformed as XML data, determining whether the second entity has a parent entity in the ER-ML hybrid data model with a third annotation marking the parent entity as not to be transformed as XML data; and in response to determining that the second entity has the parent entity with the third annotation marking the parent entity as not to be transformed as XML data: transforming the second entity and a transitive closure of the second entity to an XML column in a second table in the physical data model, the second table representing the parent entity in the ER-XML hybrid data model marked as not to be transformed as XML data; generating an XML schema document (XSD) for the XML column; and associating the XSD with the XML column in the physical data model.