Explanation

In the adopted definition, ``bi'' refers to the existence of exactly two times. An alternative definition states that a bitemporal relation has one or more system-supported valid times and one or more system-supported transaction times. In this definition, ``bi'' refers to the existence of exactly two types of times.

Most relations involving both valid and transaction time are bitemporal according to both definitions. Being the most restrictive, the adopted definition is the most desirable: It is the tightest fit, giving the most precise characterization (+E9).

The definition of bitemporal is used as the basis for applying bitemporal as a modifier to other concepts such as ``query language.'' This adds more important reasons for preferring the adopted definition.

Independently of the precise definition of bitemporal, a query language is bitemporal if and only if it supports any bitemporal relation (+E1), see Section . With the adopted definition, most query languages involving both valid and transaction time may be characterized as bitemporal. With the alternative definition, query languages that are bitemporal under the adopted definition are no longer bitemporal. This is a serious drawback of the alternative definition. It excludes the possibility of naming languages that may be precisely named using the adopted definition. With the alternative definition, those query languages have no (precise) name. What we get is a concept and name (bitemporal query language) for which there is currently little or no use.

Also, note that a query language that is bitemporal with the alternative definition is also bitemporal with regard to the adopted definition (but the adopted definition does not provide a precise characterization of this query language). Thus, the restrictive definition of a bitemporal relation results in a non-restrictive definition of bitemporal query language (and vice-versa).

We choose to name relations as opposed to databases because a database may contain several types of relations. Thus, naming relations is a more general approach.