Database Chapter 2

Entity relationship (ER) modeling

Conceptual database modeling technique

Enables the structuring and organizing of the requirements collection

process

• Provides a way to graphically represent the requirements

ER Diagram (ERD)

the result of ER modeling

entities

constructs that represent what the database keeps track of

entity instances (entity members)

occurrences of an entity, not depicted in the ER diagram, eventually recorded in the database

attribute

depiction of a characteristic of an entity represents the details that will be recorded for each entity instance

unique attribute

attribute whose value is different for each entity instance

these are underlined

relationship

ER modeling construct depicting how entities are related

cardinality constraints

depict how many instances of one entity can be associated with instances of another entity

maximum cardinality

one (represented by a straight bar: I)

many (represented by a crows foot symbol)

minimum cardinality (participation)

optional (represented by a circular symbol: 0)

mandatory (represented by a straight bar I)

composite attribute

attribute that is composed of several

uses parentheses

composite unique attribute

attribute that is composed of several attributes and whose value is different for each entity instance

multiple unique attributes

when an entity has more than one unique attribute each unique attribute is also called a candidate key

multivalued attribute

attribute for which instances o an entity can have multiple values for the same attribute

derived attribute

attribute whose values are calculated and not permanently stored in a database

optional attribute

attribute that is allowed to not have a value

exact minimum and maximum cardinality in relationships

in some cases, the exact minimum and/or maximum cardinality in relationships is known in advance

degree of a relationship

reflects how many entities involved in the relationship

binary relationship

relationship between two entities (degree 2 relationships)

unary relationship (recursive relationship)

occurs when an entity is involved in a relationship with itself (degree 1 relationship)

week entity

ER diagram constructed depicting an entity that does not have a unique attribute of its own - partial key

owner entity

entity whose unique attribute provides mechanism for identifying instances of a weak entity

identifying relationship

relationship between a weak entity and its owner entity in which each instance of a weak entity is associated with exactly one instance of an owner entity

partial key

attribute of a weak entity that, combined with the unique attribute if the owner entity uniquely identifies the weak entity’s instances

associative entity

construct used as an alternative way of depicting M:N relationships

An identical relationship represented as an M:N relationship

with an attribute and as an associative entity with an attribute