GLOSSARY - Concepts of Database Management - page 407

Subclass A class that inherits the structure and meth-
ods of another class and for which you can define addi-
tional attributes and methods.
Subquery In SQL, a query that appears within another
query.
SUBTRACT The relational algebra command for per-
forming the difference of two tables.
Superclass In UML, a class that has subclasses.
Surrogate key A system-generated primary key that is
usually hidden from users.
Synchronization The periodic exchange by a DBMS of
all updated data between two databases in a replica set.
Synthetic key A system-generated primary key that is
usually hidden from users.
Syscolumns The portion of the system catalog that
contains column information.
Sysindexes The portion of the system catalog that con-
tains index information.
Systables The portion of the system catalog that con-
tains table information.
System catalog A structure that contains information
about the objects (tables, columns, indexes, views, and
so on) in a database.
System requirement A feature that must be included in
an information system to fulfill business, legal, or user
needs.
System software The programs that control the hard-
ware and software environment. These programs
include the operating system, network managers, device
drivers, and utility programs such as sorting and data
backup.
Sysviews The portion of the system catalog that con-
tains view information.
Table See relation.
Tag A command in a Web page that a Web browser
processes to position and format the text on the screen
or to link to other files.
TCP/IP (Transmission Control Protocol and Internet
Protocol) The standard protocol for all communication
on the Internet.
Test system The hardware, software, and database that
programmers use to develop new programs and modify
existing programs. Also called a sandbox.
Thin client In a client/server architecture, a client that
performs only presentation functions.
Third normal form (3NF) A table is in third normal
form if it is in second normal form and the only deter-
minants it contains are candidate keys.
Three-tier architecture A client/server architecture in
which the clients perform the presentation functions, a
database server performs the database functions, and
the application servers perform the business functions
and serve as an interface between clients and the data-
base server. Also called an n-tier architecture.

Timestamp The unique time when the DBMS starts a
transaction update to a database.
Timestamping The process of using timestamps to avoid
the need to lock rows in a database and to eliminate the
processing time needed to apply and release locks and to
detect and resolve deadlocks.
Top-down design method A design method that begins
with a general database design that models the overall
enterprise and then repeatedly refines the model to
achieve a design that supports all necessary applications.
Transaction A set of steps completed by a DBMS to
accomplish a single user task.
Transmission Control Protocol and Internet Protocol
See TCP/IP.
Trigger An action that automatically occurs in response
to an associated database operation such as INSERT,
UPDATE, or DELETE.
Tuning The process of changing the database design to
improve performance.
Tuple The formal name for a row in a table.
Two-phase commit An approach to the commit process
in distributed systems in which there are two phases. In
the first phase, each site is instructed to prepare to
commit and must indicate whether the commit will be
possible. After each site has responded, the second
phase begins. If every site has replied in the affirmative,
all sites must commit. If any site has replied in the neg-
ative, all sites must abort the transaction.
Two-phase locking An approach to locking that is used
to manage concurrent update in which there are two
phases: a growing phase, in which the DBMS locks more
rows and releases none of the locks, and a shrinking
phase, in which the DBMS releases all the locks and
acquires no new locks.
Two-tier architecture A client/server architecture in
which the clients perform the presentation functions,
and a database server performs the database functions.
In a fat client configuration, the clients perform the
business functions, whereas in a thin client configura-
tion, the database server performs the business
functions.
UML See Unified Modeling Language.
Unified Modeling Language (UML) An approach used
to model all the aspects of software development for
object-oriented systems.
Uniform Resource Locator See URL.
Union A combination of two tables consisting of all
records that are in either table.
Union compatible Two tables are union compatible if
they have the same number of fields and if their corre-
sponding fields have identical data types.
Unnormalized relation A structure that satisfies the
properties required to be a relation (table) with the
exception of allowing repeating groups (the entries in
the table do not have to be single-valued).

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