Unlike EERM, Genera, ACEDB, and OPM, the relational model allows specifying databases in terms of relations (tables) consisting of tuples (rows), where each tuple element represents the value of a single-valued primitive attribute. Tuples are uniquely identified by primary-key attribute values and can refer to (primary-key values of) tuples in other relations via foreign-key attribute values. Objects can be represented by tuples of one or several relations, where more than one relation may be required in order to represent constructs that are not supported in relational DBMSs, such as set-valued attributes. Abstract attributes and subclass-superclass relationships are represented using foreign-key attributes, controlled value classes are represented using rules and (possibly) additional relations, while inherited attributes, derived attributes and classes are represented using SQL procedures. Object retrieval and update are also represented using SQL procedures.