9. A computer program product comprising: a computer readable storage medium having computer readable program code embodied therewith, the program code executable by a processor to: provide an entity-relationship (ER)-extensible markup language (XML) hybrid data model of a relational database comprising XML data; receive 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; receive 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; transform 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, transform the first entity marked 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, determine whether the second entity has a parent entity in the ER-XML 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 second entity has the parent entity with the third annotation marking the parent entity as not to be transformed as XML data: transform 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 data model marked as not to be transformed as XML data; generate an XML schema document (XSD) for the XML column; and associate the XSD with the XML column in the physical data model.
9. A computer program product comprising: a computer readable storage medium having computer readable program code embodied therewith, the program code executable by a processor to: provide an entity-relationship (ER)-extensible markup language (XML) hybrid data model of a relational database comprising XML data; receive 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; receive 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; transform 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, transform the first entity marked 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, determine whether the second entity has a parent entity in the ER-XML 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 second entity has the parent entity with the third annotation marking the parent entity as not to be transformed as XML data: transform 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 data model marked as not to be transformed as XML data; generate an XML schema document (XSD) for the XML column; and associate the XSD with the XML column in the physical data model. 15. The computer program product of claim 9 , wherein the program code executable by the processor further comprises: provide the physical data model comprising a fourth annotation of a first XML column marking the first XML column as not to be transformed as an entity and a fifth annotation of a second XML column marking the second XML column as to be transformed as an entity; and transform the physical data model to a second ER-XML hybrid data model, comprising: based on the fifth annotation of the second XML column marking the second XML column as to be transformed as an entity, transform the second XML column to a first entity in the second ER-XML hybrid data model; create at least a second entity in the second ER-XML hybrid data model for contents of an XSD associated with the second XML column; and based on the fourth annotation of the first XML column marking the first XML column as to be transformed to an entity, transform the first XML column as a XML attribute in a parent entity and associated with the same XSD as the second XML column.