C192 Ch4

A relation has the following 7 properties:

1) the relation has a name that is distinct from all other relation names in the relational schema; 2) each cell of the relation contains exactly one atomic (single) value; 3) each attribute has a distinct name; 4) the values of an attribute are all from the same domain; 5) each tuple is distinct; there are no duplicate tuples; 6) the order of attributes has no significance; 7) the order of tuples has no significance, theoretically. (However, in practice, the order may affect the efficiency of accessing tuples.)

Relational database

A collection of normalized relations with distinct relation names.

Domain

A domain is the set of allowable values for one or more attributes. Every attribute in a relation is defined on a domain. two or more attributes may be defined on the same domain Attribute -> postalCode DomainName -> PostalCode Meaning->The set of all possible postal codes Domain Definition -> character: size 8

Base relation

A named relation corresponding to an entity in the conceptual schema, whose tuples are physically stored in the database.

Relation schema

A named relation defined by a set of attribute and domain name pairs. Each element in the n-tuple consists of an attribute and a value for that attribute. Let A1, A2, . . . , An be attributes with domains D1, D2, . . . , Dn. Then the set {A1:D1, A2:D2, . . . , An:Dn} is a relation schema. A relation R defined by a relation schema S is a set of mappings from the attribute names to their corresponding domains. Thus, relation R is a set of n-tuples:(A1: d1, A2: d2, . . . , An:Dn) such that d1 ∊ D1, d2 ∊ D2, . . . , Dn ∊ Dn

normalized or in first normal form

A relation has the 7 properties

Candidate key

A superkey such that no proper subset is a superkey within the relation.

Tuple (record, row)

A tuple is a row of a relation.

General constraints

Additional rules specified by the users or database administrators of a database that define or constrain some aspect of the enterprise.

Superkey

An attribute, or set of attributes, that uniquely identifies a tuple within a relation.

Cartesian product

D1 and D2, where D1 = {2, 4} and D2 = {1, 3, 5} D1 × D2 D1 × D2 = {(2, 1), (2, 3), (2, 5), (4, 1), (4, 3), (4, 5)}

Entity integrity

In a base relation, no attribute of a primary key can be null.

relation instance

Set with one tuple that includes key:value pairs of attribute:value {(branchNo: B005, street: 22 Deer Rd, city: London, postcode: SW1 4EH)} a table with a column for each attribute which is atomic; each row represents an entity

Cardinality

The cardinality of a relation is the number of tuples it contains.

Degree

The degree of a relation is the number of attributes it contains.

View

The dynamic result of one or more relational operations operating on the base relations to produce another relation. A view is a virtual relation that does not necessarily exist in the database but can be produced upon request by a particular user, at the time of request.

Multiplicity Constraints

The number (or range) of possible occurrences of an entity type that may relate to a single occurrence of an associated entity type through a particular relationship. Multiplicity constrains the way that entities are related. It is a representation of the policies (or business rules) established by the user or enterprise.

intension

The structure of a relation, together with a specification of the domains and any other restrictions on possible values usually fixed, unless the meaning of a relation is changed to include additional attributes

A candidate key K for a relation R has two properties:

Uniqueness. In each tuple of R, the values of K uniquely identify that tuple. Irreducibility. No proper subset of K has the uniqueness property.

degree

a relation is the number of attributes

field is also known as

attribute, column

view

in the relational model is a virtual or derived relation that is dynamically created from the underlying base relation(s) when required

Any subset of this Cartesian product is a

relation R = {(2,1), (4,1)}

file is also known as

relation, table

extension (or state)

tuples are called the extension (or state) of a relation, which changes over time.