• Like various record-/tuple-oriented data models, many object-oriented data models have been and will be proposed.
Some of these differ only slightly in style and/or expressive power, and there is currently no clear tendency towards
one or a small number of generally "recognized" models.
• Together wi~n complex objects, applications are often concerned with object versions (multiple representations of the same semantic entity, to account for different stages, different times of validity, alternative or hypothetical information
etc.). Object-oriented database systems therefore need mechanisms to deal with versions.