Database Ch3

Relational database operator: SELECT

1. SELECT: creates a new relation by selecting only rows from an original an original relation that satisfy a specified condition.

Relationship

A linking between entities of two entity sets

Recursive Relationship

A relationship that relates an entity set to itself

Integrity Constraint Rule Def.

A rule that restricts the values in a database

Foreign key

An attribute (or combo of) in 1 relation that constitutes a primary key in another relation. Used to indicate logical links between relations

Functional Dependency

An attribute Y is functionally dependent on X if and only if each valye in column X determines one and only one value in column Y. We say X is the determinant Note: X and Y can be sets of attributes rather than single attributes. Notation: X -------> Y X determines Y Y is functionally dependent on X

Key attribute

An attribute that is part of the primary key of a relation

How are relationships between tables implemented?

By creating foreign keys

Entity Integrity

No key attribute in any row in a table may be null. The values of the primary key should be unique/distinct.

4 examples of types of domains

Numeric: 4; 2.5; Character (string): A; Ali; $ Date Logic

Tuple

Row

Primary key

The candidate key designated for principal use in uniquely identifying tuples in a relation

Degree of Relation

The number of attributes in a relation

Relation

Two dimension table of rows of data about some attributes

Null Value

Value given to an attribute in a tuple if the attribute is "inapplicable" or its value is "unknown" Not a blank or zero, it is simply unknown and may be supplied at a later time

Attribute domain

Where the attribute gets its value

Candidate key

a minimal set of attributes that uniquely identifies each row in a table. That is, a superkey that does not contain a subset of attributes that is itself a superkey

Secondary key

an attribute (or a combo of attributes) used strictly for data retrieval purposes. Ex: use of name or phone instead of customer number

Superkey

an attribute (or combination of attributes) that uniquely identifies each row in a relation Determines every other attribute in the table