The concept of user-defined time denotes time-valued attributes of database items with which the data model and query language associate no special semantics. Such attributes may have as their domains all domains that reference time, e.g., date and time. The domains may be instant-, period-, and interval-valued. For example, user-defined time attributes include “birthday,” “hiring date,” and “contract duration.” Thus, user-defined time attributes are parallel to attributes that record, e.g., salary, using domain “money” and publication count, using domain “integer.” User-defined time attributes contrast transaction-time and valid-time attributes, which carry special semantics.
The valid time and transaction time attributes of a database item are “about” the other attributes of the database item. The valid time records when the information recorded by the attributes is true in the modeled reality, and the transaction time captures when the data item was part of the...
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