Hi All, The "best practices" in RDB on 1:1 relations between entities are that the two entities E1, E2 should be merged into a single one by default, i.e., unless there is an overwhelming reason to keep them separated. Can someone provide illustrating examples of when the two entities should be merged and some of when the two should be kept separated? I think this has to see with deciding , in a given context , whether one entity can be more reasonably (within the context) when E1 can be seen as an attribute of E2 . Another reason I can think of is that we may want to keep them separate in order to restrict access to information/attributes in a given table, like, say SSN #. Thanks.