The From Clause

Information retrieval is an integral component of any database management system. Temporal database management systems should offer user-friendly and powerful means of retrieval of data according to temporal criteria. The From clause is an important component of the Select statement: it identifies the underlying relations from which the information is to be retrieved and allows the declaration of range variables. While variables merely serve as “correlation names” (e.g. for joining a table with itself) in SQL-92, TSQL2 variables are designed to increase the temporal expressiveness of the language, in addition to provide “syntactic sugar” in making some queries easier to formulate. TSQL2 range variables generalize the concept of history variables [2], which reference “groups” of tuples with a common value of a time-invariant key or surrogate, as earlier proposed for HoTQuel [1]. The main extension concerns the possibility of grouping tuples on arbitrary sets of columns. This feature also generalizes the concept of a restructuring operator [3], which changes the key of a temporal relation, as proposed in TempSQL [4].