Introduction to Database Management System

DATABASEMODELSDATABASEMODELS

Database models A database model

◦ defines the logical design of data.◦ Describes the relationships between different

parts of data. 4 models

◦ Entity relationship model◦ Hierarchical model◦ Network model◦ Relational model

Entity Relationship modeling IEEE defines ER diagram as ‘ a diagram that depicts a set of real-world

entities and the logical relationships among them’

It contains:◦ Data Objects (Entities)◦ Attributes◦ Relationships

Entity Types Definition:

◦ An entity is an object or event, real or abstract, about which we would like to store data. Entity is the abbreviation of entity type. It represent a set of entity instances which can be described by the same set of attribute types. The value of the same attribute for each entity instance may be different.

Properties of Entity TypesAn entity has the following Name Description Identifier Properties: Estimated number (Max., Min.,

Average) of entity instances entity instances Attributes that describe the Entity Types Examples of entity type instances

Entity Type and Entity Instance (Occurrence)

Entity Types Entity InstanceVendor ABC Co.Employee TarunCourse MBADepartment Marketing Department

Where to Look for an Entity Type? Tangible or Intangible Things

◦ The nouns that are used to describe the problem domain will often correspond to the major Entity Types of the system.

◦ Examples: Product, Sensor, and Employee, Department, and Sale Office.

Resources◦ Any resources that an organization needs to manage should be

represented as an Entity Type. ◦ Examples: Inventory, Machine, Bank Account, and Customer.

Roles Played◦ Roles can be played by persons or organizational units. ◦ Examples: Customers, Managers, and Account representatives.

Events◦ Events are incidents that occur. An event often involve an

interaction between two Entity Types or an action that changes the status of an Entity Type.

◦ Examples: Sale, Delivery, and Registration of a motor vehicle.

Data Attributes describe the properties of a data object. Attributes that identify entities are known as key attributes, that describes an entity are known as non-key attributes.Data attributes are used to perform the following function

Naming the instance of the data object

Description of instance

Attributes

ATTRIBUTE

an entity has a set of attributes attribute defines property of an entity it is given a name attribute has value for each entity value may change over time

Relationships The connection of data objects or entities

with each other is known as relationship.

car

person car

person

Basic connection between data objects

owns

Relationship between data objects

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Degree of Relationships

Degree of a relationship is the number of entity types that participate in it◦Unary Relationship◦Binary Relationship◦Ternary Relationship

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Degree of relationships – from Figure 3-2

Entities of two different types related to each other Entities of three

different types related to each other

One entity related to another of the same entity type

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Figure 3-12 Examples of relationships of different degrees

a) Unary relationships

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Figure Examples of relationships of different degrees (cont.)

b) Binary relationships

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Examples of relationships of different degrees (cont.)

c) Ternary relationship

Note: a relationship can have attributes of its own

Cardinality Cardinality is the specification of the

number of occurrences (instances) of one object that relates to the number of instances of another object.

One-to one relationship One-to many relationship Many-to-many relationship

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Cardinality of Relationships One-to-One

◦ Each entity in the relationship will have exactly one related entity

One-to-Many◦ An entity on one side of the relationship can

have many related entities, but an entity on the other side will have a maximum of one related entity

Many-to-Many◦ Entities on both sides of the relationship can

have many related entities on the other side

Relationship CardinalityE1 E2

E1

E1

E2

E2

One-to-ManyOne-to-Many

Many-to-ManyMany-to-Many

One-to-OneOne-to-One

1:1

1:M

M:N

Relationship Cardinality The number of Entity Instances involved in the

Relationship Instances Grouping in a Relationship Type.

Three Forms of Cardinality 1.One-to-one (1:1)

DEPARTMENT has MANAGEREach DEPARTMENT has one and only one MANAGEREach MANAGER manages one and only one DEPARTMENT

2.One-to-many (1:m)CUSTOMER places ORDEREach CUSTOMER sometimes (95%) place one or more ORDERsEach ORDER always is placed by exactly one CUSTOMER

3.Many-to-many (m:n)INSTRUCTOR teaches COURSE Each INSTRUCTION teaches zero, one, or more COURSEsEach COURSE is taught by one or more INSTRUCTORs

Modality Modality describes the possibility that

whether a relationship between the entities is required or not.

The modality of a relationship is 0 if the relationship is optional, and the modality is 1 if an occurrence of the relationship is essential.

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Figure Examples of cardinality constraints

a) Mandatory cardinalities

A patient must have recorded at least one history, and can have many

A patient history is recorded for one and only one patient

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Figure Examples of cardinality constraints (cont.)

b) One optional, one mandatory

An employee can be assigned to any number of projects, or may not be assigned to any at all

A project must be assigned to at least one employee, and may be assigned to many

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Sample E-R Diagram (Figure )

E-R Diagram : Examples

Modality: the customer is required to have an order

Modality: the customer can arrive without any order

Cardinality: single customerPlaced a given order

Cardinality: customer has Many order

customer order

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Strong vs. Weak Entities, andIdentifying Relationships

Strong entities ◦ exist independently of other types of entities◦ has its own unique identifier◦ identifier underlined with single-line

Weak entity◦ dependent on a strong entity (identifying owner)…cannot

exist on its own◦ does not have a unique identifier (only a partial identifier)◦ Partial identifier underlined with double-line◦ Entity box has double line

Identifying relationship◦ links strong entities to weak entities

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Strong entity Weak entity

Identifying relationship