In a query or update statement, temporal projection pairs the computed facts with their associated times, usually derived from the associated times of the underlying facts.
The generic notion of temporal projection may be applied to various specific time dimensions. For example, valid-time projection associates with derived facts the times at which they are valid, usually based on the valid times of the underlying facts.
While almost all temporal query languages support temporal projection, the flexibility of that support varies greatly.
In some languages, temporal projection is implicit and is based the intersection of the times of the underlying facts. Other languages have special constructs to specify temporal projection.
The term “temporal projection” has been used extensively in the literature. It derives from the retrieve clause in Quel as well as the SELECTclause in SQL, which both serve the purpose of the relational algebra...
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