In
temporal database
A temporal database stores data relating to time instances. It offers temporal data types and stores information relating to past, present and future time.
Temporal databases could be uni-temporal, bi-temporal or tri-temporal.
More specifically th ...
s, valid time (VT) is the time period during which a database fact is valid in the modeled reality. As of December 2011, ISO/IEC 9075, Database Language
SQL:2011 Part 2: SQL/Foundation included clauses in table definitions to define "application-time period tables" (that is, valid-time tables). ''Valid time'' was coined by
Richard T. Snodgrass and his doctoral student.
[Richard T. Snodgrass and Ilsoo Ahn, "Temporal Databases," IEEE Computer 19(9), September, 1986, pp. 35-42.]
In a database table valid time is often represented by two extra table-columns ''StartVT'' and ''EndVT''. The time interval is closed at its
lower bound
In mathematics, particularly in order theory, an upper bound or majorant of a subset of some preordered set is an element of that is greater than or equal to every element of .
Dually, a lower bound or minorant of is defined to be an element ...
and open at its
upper bound
In mathematics, particularly in order theory, an upper bound or majorant of a subset of some preordered set is an element of that is greater than or equal to every element of .
Dually, a lower bound or minorant of is defined to be an element ...
.
Example:
Valid time is the time for which a fact is true in the real world. In the example above, the Person table gets two extra fields, Valid-From and Valid-To, specifying when a person's address was valid in the real world. On April 4, 1975, John's father proudly registered his son's birth. An official will then insert a new entry to the database stating that John lives in Smallville from April, 3rd. Notice that although the data was inserted on the 4th, the database states that the information is valid since the 3rd. The official does not yet know if or when John will ever move to a better place so in the database the Valid-To is filled with infinity (∞). Resulting in this entry in the database:
December 27, 1994 John reports his new address in Bigtown where he has been living since August 26, 1994. The Bigtown official does not change the address of the current entry of John Doe in the database. He adds a new one:
The original entry Person (John Doe, Smallville, 3-Apr-1975, ∞) is then updated (not removed!). Since it is now known that John stopped living in Smallville on August 26, 1994, the Valid-To entry can be filled in. The database now contains two entries for John Doe
When John dies the database is once more updated. The current entry will be updated stating that John does not live in Bigtown any longer. No new entry is being added because officials never report heaven as a new address. The database now looks like this
See also
*
Transaction time
*
Decision time
*
Slowly changing dimension
A slowly changing dimension (SCD) in data management and data warehousing is a dimension which contains relatively static data which can change slowly but unpredictably, rather than according to a regular schedule. Some examples of typical slowly ...
References
Database management systems
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