Although “
now
” is expressed in SQL and CURRENT_TIMESTAMP within queries, this value cannot be stored in the database. How ever, this notion of an ever-increasing current-time value has been reflected in some temporal data models by inclusion of database-resident variables, such as “
now
” “
until-changed,
” “**,” “@,” and “-”. Time variables are very desirable, but their used also leads to a new type of database, consisting of tuples with variables, termed a
variable database.
This glossary contains concepts specific to temporal databases that are well-defined, well understood, and widely used. In addition to defining and naming the concepts, the glossary also explains the decisions made. It lists competing alternatives and discusses the pros and cons of these. It also includes evaluation criteria for the naming of concepts.This paper is a structured presentation of the results of e-mail discussions initiated during the preparation of the first book on temporal databases, Temporal Databases: Theory, Design, and Implementation, published by Benjamin/Cummings, to appear January 1993. Independently of the book, an initiative aimed at designing a consensus Temporal SQL is under way. The paper is a contribution towards establishing common terminology, an initial subtask of this initiative.
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