Information Management (Chapter 5)

Determination

The role of a key. In the context of a database table, the statement “A determines B” indicates that knowing the value of attribute A means that the value of attribute B can be looked up.

Functional dependence

Within a relation R, an attribute B is functionally dependent on an attribute A if and only if a given value of attribute A determines exactly one value of attribute B.

Determinant

Any attribute in a specific row whose value directly determines other values in that row

Dependent

An attribute whose value is determined by another attribute

Full functional dependence

A condition in which an attribute is functionally dependent on a composite key but not on any subset of the key.

Flags (Integrity Rules)

Special codes implemented by designers to trigger a required response, alert end users to specified conditions, or encode values. • Flags may be used to prevent nulls by bringing attention to the absence of a value in a table.

Composite entity (M:N Relationship)

An entity designed to transform an M:N relationship into two 1:M relationships. The composite entity’s primary key comprises at least the primary keys of the entities that it connects. • Also known as a bridge entity or associative entity.

Linking table (M:N Relationship)

In the relational model, a table that implements an M:M relationship.

Index

An ordered array of index key values and row ID values (pointers). Indexes are generally used to speed up and facilitate data retrieval. • Also known as an index key.

Unique index

An index in which the index key can have only one associated pointer value (row).

Relational algebra

A set of mathematical principles that form the basis for manipulating relational table contents

Relvar

Short for relation variable, a variable that holds a relation. A relvar is a container (variable) for holding relation data, not the relation itself

closure

A property of relational operators that permits the use of relational algebra operators on existing tables (relations) to produce new relations

SELECT

In relational algebra, an operator used to select a subset of rows. • Also known as RESTRICT

PROJECT

In relational algebra, an operator used to select a subset of columns.

UNION

In relational algebra, an operator used to merge (append) two tables into a new table, dropping the duplicate rows. The tables must be union-compatible.

Union-compatible

Two or more tables that have the same number of columns and the corresponding columns have compatible domains

INTERSECT

In relational algebra, an operator used to yield only the rows that are common to two union-compatible tables.

DIFFERENCE

In relational algebra, an operator used to yield all rows from one table that are not found in another union-compatible table.

PRODUCT

In relational algebra, an operator used to yield all possible pairs of rows from two tables. • Also known as the Cartesian product.

DIVIDE

In relational algebra, an operator that answers queries about one set of data being associated with all values of data in another set of data.

Data dictionary

A DBMS component that stores metadata—data about data

System catalog

A detailed system data dictionary that describes all objects in a database.

Homonym

The use of the same name to label different attributes. Homonyms generally should be avoided.

Synonym

The use of different names to identify the same object, such as an entity, an attribute, or a relationship; synonyms should generally be avoided