Upward compatibility aids in the smooth migration of applications from a conventional to a temporal data model. The definitions introduced here allow a specific temporal language to be evaluated as to the degree that it ensures upward compatibility. The extant temporal extensions to SQL are all deficient in one or more ways, rendering migration more difficult. We subsequently showed how SQL-92 can be extended to yield a temporal data model satisfying all three notions of upward compatibility. Applications can be much more easily migrated to this new data model. The notion of temporal upward compatibility can be viewed as a form of logical data independence. In the same way that an external schema can ensure that applications are not impacted by changes to the logical schema, temporal upward compatibility ensures that applications are not impacted by a specific kind of change to the logical schema: adding or removing temporal support. Logical data independence is an important benefit provided by modern data models, in particular by the relational data model, and the specific kind discussed here provides similar advantages.
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