Entity RELATIONSHIP MODEL | INFORMATION MANAGEMENT

Entities

It is a person, place object, event or a concept in the user environment about which the organization wishes to maintain data.

Entity Type

IT is a collection of entities that share common properties or characteristics. Each entity type in an E-R model is given a name which is placed inside a box representing the entity type.

Entity Instance

Is a singular occurrence of an entity type.

Example: There is one EMPLOYEE entity type in most organizations, but there may be hundreds (or even thousands ) of instances of this entity type stored in the database.

Strong Entity Type

An entity type that exists independently of other entity types.

Strong Entity Type

Always have a unique characteristic called an identifier.

Strong Entity Type

Examples include: STUDENT, EMPLOYEE, CLIENT, PATIENT, COURSE

Weak Entity Type

An entity type whose existence depends on some other entity type.

Weak Entity Type

it has no business meaning in the E-R diagram without the entity on which it depends.

identifying relationship

The relationship between a weak entity and its identifying owner is called

identifying owner

the entity type on which the weak entity depends in called

Attributes

property or characteristic of an entity type that is of interest to the organization.

Simple Attribute

is an attribute that cannot be broken down into smaller components.

Simple Attribute

All of the attributes associated with AUTOMOBILE are simple: Vehicle_ID, Color, Weight.

Composite Attribute

is an attribute that can be broken down into component parts such as Address.

Symbol: Address

Single_Valued Attribute

An attribute that has only one value.

Multivalued Attribute

An attribute that may take on more than one value for a given entity instance.

derived Attribute

An attribute whose values can be calculated from related attribute values (plus possibly data not in the database, such as today’s date, current_time or a security code provided by the system user.)

Identifier attribute

It is an attribute that uniquely identifies individual instances of an entity type.

STUDENT : Student_ID

EMPLOYEE : Employee_ID

Relationship

t is an association among the instances of one or more entity types that is of interest to the organization.

Are the glue that holds together the various components of an E-R model.

Relationship Type

Is a meaningful association between (or among) entity types.

Relationship type

It is denoted by a diamond symbol containing the name of the relationship

degree

relationship is the number of entity types that participate in that relationship.

Unary relationships (Degree 1)

Binary relationships (Degree 2)

Ternary relationships (Degree 3)

The three (3) most common relationship degrees in E-R model are

Unary

It is a relationship between the instances of a single entity type.

unary

It is also called as “recursive relationship”.

binary

Is a relationship between the instances of two entity types and is the most common type of relationship encountered in data modeling.

ternary Relationship

It is a simultaneous relationship among the instances of three(3) entity types.

Cardinality Constrains

Specifies the number of instances of one entity that can(or must) be associated with each instance of another entity.

Minimum Cardinality

It is the minimum number of instances of entity B that may be associated with each instance of entity A.

In the example, the minimum number of videotapes for a movie is 0. If it is 0, entity type B is an optional participant in the relationship.

Maximum Cardinality

It is the maximum number of instances of entity B that may be associated with each instance of entity A.