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Valid-Time Selection and Projection in TSQL2

Temporal databases have now been studied for more than a decade. During that period of time, numerous query languages have been proposed for temporal databases. One of the essential components of a temporal query language is valid-time selection, which allows the user to retrieve tuples according to their valid-time relationship. Often, this component is closely tied to another important component, valid-time projection, which defines the timestamps of the tuples in query results. Here, nine different temporal query languages, primarily SQL and QUEL extensions, are examined with a focus on valid-time selection and projection. Based on that survey, the specific design of the valid-time selection and projection components of the consensus temporal query language TSQL2 are presented.