On the semantics of “now” in databases

ABSTRACT

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.

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INDEX TERMS

Primary Classification:
  H. Information Systems
  H.2 DATABASE MANAGEMENT
      H.2.3 Languages

          Nouns: SQL

Additional Classification:
  H. Information Systems
  H.2 DATABASE MANAGEMENT
      H.2.3 Languages
          Subjects: Data description languages (DDL)
      H.2.4 Systems
          Subjects: Query processing

General Terms:
Languages, Performance

Keywords:
Now, SQL, TSQL2, indeterminacy, now-relative value, temporal query languge