Database Reference
In-Depth Information
Surrogate key.
A unique value generated by the computer system used as a key for
a relation. A surrogate key has no business meaning apart from the computer
system.
Synonyms.
Two or more data elements containing the same data but having dif-
ferent names.
Third normal form (3NF).
A relation that is already in the second normal form and
without any transitive dependencies—that is, the dependency of a nonkey
attribute on the primary key through another nonkey attribute, not directly.
Transaction.
Database operations that are executed as a unified set on the data in
the database. For example, the transfer of money from the savings to the check-
ing account involving database changes to two relations is considered as a
transaction.
Transaction manager.
Software component in the DBMS that enables the proper
execution of transactions.
Transitive dependency.
In a relation, the dependency of a nonkey attribute on the
primary key through another nonkey attribute, not directly.
Tuple.
A row in a relational table.
Two-phase commit.
A technique for concurrency control in a distributed database
environment. Commit of a transaction happens in two phases—first, the coordi-
nating DBMS sends updates to other databases asking them to execute; next,
coordinating the DBMS sends messages to commit after participating databases
agree on the updates.
Vertical partitioning.
In a relational data model, the division of a relation vertically
as sets of columns.
VSAM (virtual sequential access method).
An improved data access method in file-
oriented data systems.
Weak entity.
An entity that depends on another entity for its existence. For
example, the entity type ORDER DETAIL cannot exist without the entity type
ORDER.
