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Definition A temporal database contains time-referenced, or timestamped, facts. A time reference in such a database is absolute if its value is independent of the context, including the current time, now.

Abstract Versus Concrete Temporal Query Languages J AN C HOMICKI 1, DAVID T OMAN 2 1 State University of New York at Buffalo, Buffalo, NY, USA 2 University of Waterloo, Waterloo, ON, Canada

Synonyms Historical query languages

Definition Key Points An example is ‘‘Mary’s salary was raised on March 30, 2007.’’ The fact here is that Mary’s salary was raised. The absolute time reference is March 30, 2007, which is a time instant at the granularity of day. Another example is ‘‘Mary’s monthly salary was $ 15,000 from January 1, 2006 to November 30, 2007.’’ In this example, the absolute time reference is the time period [January 1, 2006
November 30, 2007]. Absolute time can be contrasted with relative time.

Cross-references ▶ Now in Temporal Databases ▶ Relative Time ▶ Time Instant ▶ Time Period ▶ Temporal Database ▶ Temporal Granularity

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Bettini C., Dyreson C.E., Evans W.S., Snodgrass R.T., and Wang X.S. A glossary of time granularity concepts. In Temporal Databases: Research and Practice. O. Etzion, S. Jajodia, S. Sripada (eds.). LNCS, vol. 1399. Springer, 1998, pp. 406–413. Jensen C.S. and Dyreson C.E. (eds.). A consensus glossary of temporal database concepts – February 1998 version. In Temporal Databases: Research and Practice, O. Etzion, S. Jajodia, S. Sripada (eds.). LNCS, vol. 1399. Springer, 1998, pp. 367–405. 2009 Springer ScienceþBusiness Media, LLC

Temporal query languages are a family of query languages designed to query (and access in general) timedependent information stored in temporal databases. The languages are commonly defined as extensions of standard query languages for non-temporal databases with temporal features. The additional features reflect the way dependencies of data on time are captured by and represented in the underlying temporal data model.

Historical Background Most databases store time-varying information. On the other hand, SQL is often the language of choice for developing applications that utilize the information in these databases. Plain SQL, however, does not seem to provide adequate support for temporal applications. Example. To represent the employment histories of persons, a common relational design would use a schema EmploymentðFromDate;ToDate; EID; CompanyÞ;

with the intended meaning that a person identified by EID worked for Company continuously from FromDate to ToDate. Note that while the above schema is a standard relational schema, the additional assumption that the values of the attributes FromDate and ToDate represent continuous periods of time is itself not a part of the relational model. Formulating even simple queries over such a schema is non-trivial. For example, the query GAPS: “List all persons with gaps in their employment history, together

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Abstract Versus Concrete Temporal Query Languages

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