Valid-Time Selection and Projection

Temporal database management systems should offer user-friendly and powerful means of retrieval of data according to temporal criteria. Selection of tuples according to their valid times, which is the times when the information represented by the tuples is valid in the modeled reality [16], is termed valid-time selection. The attributes to be computed and returned from a query are specified in the target list. Valid-time projection is the temporal analog of regular projection in, e.g., Quel, and allows for the specification of the implicit timestamps for tuples of the resulting relation. Valid-time projection mechanisms may be partitioned into two categories. First, valid-time projection may be built implicitly into the operators and constructs of the query language. This leaves the user no freedom in specifying the timestamps of result tuples. Second, valid-time projection may be specified explicitly, as part of an existing clause or in a new clause. This approach allows the user to control the timestamps of result tuples. The chapter is structured as follows. Initially, we survey previous temporal query language proposals. We then describe six general criteria for good language design. The criteria provide guidelines. However, they may conflict at times, and a single general criterion may be applied in several ways in a concrete design. Then, TSQL2’s method for valid-time selection is introduced. Mechanisms that involve the referencing and construction/extraction of various kinds of timestamps are covered first. Then, comparison operators are covered. Finally, miscellaneous aspects of the design are discussed. The general design is first presented, and then central design issues are discussed in more detail. We then turn to temporal projection. The motivation for adding a separate clause for valid-time projection is discussed, and the choice of defaults is considered