Developing Time-Oriented Database Applications in SQL
An enjoyable plunge into the how-to of time and data, with real-world situations and code.
TABLE OF CONTENTS Developing Time-Oriented Database Applications in SQL
Chapter 10: Bitemporal Tables
OVERVIEW
A bitemporal table is a glorious structure. It simultaneously records the history of the enterprise, while also capturing the sequence of changes to the record of that history. Bitemporal tables permit queries on the history . Bitemporal tables permit queries on the history as best known (over valid time, with a transaction time of "now"), queries on the change history of a stored data item (over transaction time, with a fixed valid time), and queries on the interaction of valid time and transaction time (for example, finding that information stored retroactively, after the fact). It is this range of queries that makes bitemporal tables so versatile and useful.
Bitemporal tables, in capturing both valid time and transaction time, require care in maintaining. Modifications must not disturb previously recorded information; they must be append-only. As such, modifications generally require longer sequences of more complex SQL statements.
Bitemporal tables admit a wide variety of temporal queries and integrity constraints. They also admit a variety of representational schemes, which speed up some common queries at the expense of some rarer queries.
Information is the key asset of many companies. Nykredit, a major Danish mortgage bank, is a good example. In 1989, the Danish legislature changed the Mortgage Credit Act to allow mortgage providers to market loans directly to customers and through real estate agents; before such loans had to be funneled indirectly through banks, with the mortgage providers separated from the consumer by these middlemen.
This change in the law had...
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