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등록일 2004.09.15 HIT 343226 작성자 폰돌

Glossary of Database Terminology


ad hoc query
a temporary query for which neitherthe specification nor the output are saved.

attribute
describes a value whichwill be found in each tuple in a relation. Usually represented as a column of a relation.

base query
same as ground query

BCNF
Boyce-Codd Normal Form

Boyce-Codd dependency
a (part of the) key is dependent on a non-key attribute

Boyce-Codd Normal Form
relations are in third normal form and there are no Boyce-Codd dependencies.

candidate key
one or more attributes which will uniquely identify one tuple in a relation. A candidate key is a potential primary key.

commit
decision to proceed with the actual posting of a change to the database.

computed attribute
an attribute for which the valueis calculated from other attributes. While computed attributes may be stored in relations, they are not usually. Most often they are produced as needed in the resolution of a query.

criterion (pl. criteria)
a characteristic or limitationapplied to the values of an attribute to select some of the tuples of a relation during a query.

database
1). a collection of all the data needed by a person or organization to perform needed functions
2). a collection of related files
3). any collection of data orgainzed to answer queries
4). (informally) a database management system

database manager
1). the person with primary responsibilty for the design, construction, and maintenance of a database.
2). (informally) a database management system.

database management system
- also called a database manager. an integrated collection of programs designed to allow people to design databases, enter and maintain data, and perform queries

deadlock
a pathological state of a computer system reached when none of a group of competing processes can proceed because each is waiting for resources locked by the other(s).

distributed database
A database in which the resources are stored on more than one computer system, often at different physical locations.

domain
the collection of all possible values of an attribute. The domain of an attribute may be finite or infinite.

entity
a real-world object, observation, transaction, or person about which data are to be stored in a database.

entity-relationship (ER) diagram
design tool used primarily for relational databases in which entities are modeled as geometric shapes and the relationships between them are shown as labeled arcs

field
term used by Access as a synonym forattribute.

fifth normal form
A relation is in fifth normal form if it is in fourth normal form and it cannot be decomposed into smaller realtions without loss of information

file
1). the separately-named unit of storage for all data and programs on most computers. For example, a relation or a whole database may be stored in one file.
2). term used as a synonym for relationin some (particularly older) database managers, like dBase.

filter
Access term for persistent query

first normal form
relations have no multiple-valued attributes

fourth normal form
A relation is in fourth normal form if it is Boyce-Codd normal form and it has at most only one independent multivalued dependency.

granularity
the size of the smallest unit which canbe independently locked. A database may apply locks at the database level,the relation level, the tuple level, or on the value of a single attribute within a tuple.

ground query
a query in which all of the attributes aretaken directly from relations; none are taken from the output of other queries.

identity
a relation in which the attributes.

index
1). a method used to reorder tuples or to displaythem in a specific order
2). a data structure used to give rapid, random access to relations. Indexes are most often used with large relations.

inversion
an index from which the tuples in a main relation with a particular value for an attribute can be determined

join
a query which uses data from more than onerelation. The relations must have at least one attribute (called thejoin or linking attribute) in common.

key
an attribute or combination of attributes.A combination of their values will be used to select tuples from a relation.

locking
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.

many-to-many relationship
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.

negation
specifying that a query is to select all tuplesexcept those which have a particular value for a given attribute.Such queries are often unsafe, and many databases do not support negation.

normal form
1). a condition of relations and databasesintended to reduce data redundancy and improve performance
2). rules and processes for putting relation and databases into normal form

one-to-many relationship
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.

one-to-one relationship
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.

parital dependency
the value of a non-key attribute isdependent on only part of the key (usually on one attribute of a multi-attribute key)

persistent query
a query which is stored for reuse

post
to make a change to the storeddata. Posting may be done immediately or may be deferred until all relatedchanges can be checked and verified. See commit.

primary key
a key such that thevalue of the key attribute(s) will uniquely identify any tuple in the relation. A relation must not have more than one primary key.

primary memory
The internal memory of a computer, usually consisting of RAM chips, on which the data and programs currently being used must reside.

project
a query in which only some of the attributes in thesource relation appear in the output

QBE
Query-By-Example, a tabular language for expressingqeuries, in which a query is specified by sketching an abbreviated'schema' of theoutput desired, along with any criteria which should be applied to the attributes desired.

query
literally, a question.
1). a command, written in a query language, for the database to present a specified subset of the data in the database.
2). the subset of data produced as output in response to a query

query-by-example
see QBE

query language
a computer language which can be used to express queries.

query resolution
the process of collecting the data needed to answer a query

read lock
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.

record
term used as a synonym for tuplein some (particularly older) database management systems, like dBase.

recursive query
a query in which the output of the queryis then used as input for the same query.


relation
the basic collection of data in a relational database. Usually represented as a rectangualar array of data, in which each row (tuple) is a collection of data about one entity

row
term used by Access as a synonym for tuple

running a query
Access term for query resolution

safe query
a query in which the output is guaranteedto be finite.

schema
1).a description of a database. It specifies (among other things) the relations, their attributes, and the domains of the attributes. In some database systems, the join attributes are also specified as part of the schema.
2). the description of one relation

secondary memory
The external storage of a computer, usually consisting of magnetic or optical disks, on which data and programs are held between the times they are actually in use.

second normal form
relations are in first normal form andhave no partial dependencies

secondary key
a key which is not the primary key for a relation.

select
a query in which only some of the tuples in the source relation appear in the output

Sequel
see SQL

Sequential Query Language
see SQL

SQL
pronounced 'Sequel', stands for Sequential Query Language, the most common text-based database query language

table
term used by Access as a synonym for relation

third normal form
relations are in second normal form and have no transitive dependencies

transaction
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.

transitive dependency
the value of one non-key attributeis dependent on the value of one or more other non-key attributes

tuple
within a relation, a collection of all the facts related to one entity. Usually represented as a row of data.

two-phase commmit
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

unsafe query
a query in which the output is possiblyinfinite. This most often occurrs in queries which are recursive orwhich contain negation. Such queries are disallowed by many database managment systems to prevent unsafe queries.

value
the computer representation of a fact about an entity

write lock
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.


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last modified 4/9/96
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