reserving the use of a database, relation, tuple,or other collection of data for access by one user. This is a strategyto prevent anomalous behaviour of a database as would happen, for example,when one user reads data from a database which was in the process of beingmodified by another user.
One or more tuples in one relationmay be related to one or more tuples in a second relation by a commonvalue of a join attribute. This implies that each value of the join attribute may appear any number of times in either relation or in both.
exactly one tuple in one relation is related by a common join attribute to many tuples in another relation. This implies that each value of the join attribute is unique in the first relation but not necessarily unique in the second.
exactly one tuple in one relation is related by a common join attribute to exactly one tuple in another relation.This implies that each value of the join attribute appears no more than once in each of the relations.
a lock which allows the user receiving the lock to read all or part of a database but not to modify it. Some database management systems will allow many users to have simultaneous read locks on a unit of data.
1). the fundamental unit of change in many (transaction-oriented) databases. A single transaction may involvechanges in several relations, all of which must be made simultaneouslyin order for the database to be internally consistent and correct.
2). the real-life event which is modeled by the changes to the database.
a strategy in which changes to a databaseare made temporarily to a buffered version of the database. Onceit has been determined that all parts of an update can be made successfully,the changes are posted to the actual database
a lock which allows the user receiving thelock to read and to modify the database. Write locks almost always implyexclusive control of the database; no other users will be allowed to haveeither read or write locks as long as one write lock is active.
maintained by rjm last modified 4/9/96 This page has been accessed 1175 times.