Chapter 2 Entity-Relationship Model 2.1 IntroductionIntroduction 2.2 Basic conceptsBasic concepts 2.3 mapping constrainsmapping constrains 2.4 keyskeys

Chapter 2 Entity-Relationship Model

2.1 Introduction 2.2 Basic concepts 2.3 mapping constrains 2.4 keys 2.5 Entity Relationship Diagram 2.6 Weak Entity Sets 2.7 Extended E-R Features 2.8 Reduction of an E-R Schema to Table

2.1 Introduction

We can characterize the overall approach to the semantic modeling problem in terms of the following four steps:

First, identify a set of semantic concepts. ① we might agree that the world is made up of entities.

② we might go further and agree that entities can usefully be classified into entity types.

③ we might go still further and agree that every entity has a special property that severs to identify that entity.

④ we might go further again and agree that any entity can be related to other entities by means of relationships.

2.1 Introduction

Next we try to devise a set of corresponding symbolic objects that can be used to represent the foregoing semantic concepts.

We also devise a set of formal, general integrity rules to go along with those formal objects.

Finally, we also develop a set of formals operators for manipulating those formal objects.

2.2 Basic Concepts

There are three basic notions that the E-R data model employs: entity sets, relationship sets, and attributes.

1.Entity Sets An entity is a “thing ” or “object” in the real world that is

distinguishable from all other objects. An entity has a set of properties, and the values for some set of properties may uniquely identify an entity, an entity may be concrete or it may be abstract.

An entity set is a set of entities of the same type that share the same properties, or attributes.

Entity set do not need to be disjoint. An entity is represented by a set of attributes. For each attribute,

there is a set of permitted values, called the domain, or value set, of that attribute.

2.2 Basic Concepts

An attribute, as used in the E-R model, can be characterized by the following attribute types:

1)simple and composite attributes

the simple attributes are not divided into subparts

composite attributes can be divided into subparts

customer-name

first-name middle-initial last-name

customer-address

street statecity postal-code

street-number street-name apartment-number

Example:

Figure 2.2 composite attributes customer-name and customer-address

2.2 Basic Concepts

2)single-valued and multivalued attributes the attributes that have a single value for a particular entity is called single-valued attributes. an attribute that has a set of values for a specific entity is called multivalued attributes. Example: phone-number, dependent-name3)null attributes A null value is used when an entity does not have a value for an attribute. an attribute value is unknown : missing (name) or not known(apartment-number)4)derived attribute the value for this type of attribute can be derived from the values of other related attributes or entities. age=date–date-of-birth

2.2 Basic Concepts

2.Relationship Sets A relationship is an association among several entities. A relationship set is a set of relationships of the same type. A relationship set is a mathematical relation on n≥2 entity sets. If E1,E2,……En are entity sets, then a relationship set R is a subset of

{(e1,e2,……en) ︱ e1 E1, e2 E2…… en En}∈ ∈ ∈ The entities involved in a given relationship are said to be participants in that relationship. The number of participants in a given relationship is called the degree of that relationship.

335-57-7991 Adams Spring Pittsfield

321-12-3123 Jones Main Harrison

019-28-3746 Smith North Rye

677-89-9001 Hayes Main Harrison

555-55-5555 Jackson Dupont Woodside

244-66-8800 Curry North Rye

963-96-3969 Williams Nassau Princeton

L-23 1000

L-17 1000

L-15 1500

L-14 1500

L-19 500

L-11 900

L-16 1300

customer loan

Figure 2.3 Relationship set borrower

2.3 Mapping constrains

An E-R enterprise schema may define certain constraints to which the contents of a database must conform.

1.Mapping cardinalities mapping cardinalities, of cardinality ratios, express the number

of entities to which another entity can be associated via a relationship set.

For a binary relationship set R between entity sets A and B, the mapping cardinality must be one of the following:

1)one to one : an entity in A is associated with at most one entity in B, and entity in B is associated with at most one entity in A.

2)one to many: an entity in A is associated with any number of entities in B. An entity in B, however, can be associated with at most one entity in A.


3)many to one: An entity in A is associated with at most one entity in B. An entity in B, however, can be associated with any number of entities in A.

4)many to many: An entity in A is associated with any number of entities in B, and an entity in B is associated with any number of entities in A.

a2

a3

a4

a1

b2

b3

b4

b1

A B

One to one

b2

b3

b4

b5

b1

a2

a3

a1

A B

One to many

b2

b3

b1a2

a3

a4

a5

a1

A B

Many to one

a2

a3

a4

a1

b2

b3

b4

b1

A B

Many to many

2. Existence Dependencies

If the existence of entity x depends on the existence of entity y, then x is said to be existence dependent on y. Operationally, if y is deleted, so is x. Entity y is said to be a dominant entity, and x is said to be a subordinate entity.

The participation of an entity set E in a relationship set R is said to be total if every entity in E participates in at least one relationship in R. If only some entities in E participate in relationship in R, the participation of entity set E in relationship R is said to be partial.


2.4 key

A key allows us to identify a set of attributes that suffice to distinguish entities from each other. (relationships)

1. Entity Sets

superkey : superkey is a set of one or more attributes, that, taken collectively, allows us to identify uniquely an entity in the entity set.

candidate keys: minimal superkeys

primary key: a candidate key that is chosen

by the database designer as the principal means

of identifying entities within an entity set.

superkey

candidate keys

primary key

2.4 key

2. Relationship Sets Let R be a relationship set involving entity sets E1,E2,……En. Let

primary-key(Ei) denote the set of attributes that forms the primary key for entity set Ei.

The composition of the primary key for a relationship set depends on the structure of the attributes associated with the relationship set R.

If the relationship set R has no attributes associated with it, then the set of attributes

primary-key(E1) primary-key(E∪ 2) …… primary-key(E∪ ∪ n) describes an individual relationship in set R.

2.4 key

If the relationship set R has attributes a1,a2,……an associated with if, then the set of attributes.primary-key(E1) primary-key(E∪ 2) …… primary-key(E∪ ∪ n) {a1,a2,……an }∪ describes an individual relationship in set R.

In both of the above cases, the set of attributes. primary-key(E1) primary-key(E∪ 2) …… primary-key(E∪ ∪ n)

from a superkey for the relationship set.

The structure of the primary key for the relationship set depends on the mapping cardinality of the relationship set.

2.5 Entity Relationship Diagram

A E-R diagram consists of the following major components: 1. Rectangles, which represent entity sets 2. Ellipses, which represent attributes 3. Diamonds, which represent relationship sets 4. Lines, which link attributes to entity sets and entity sets to

relationship sets 5. Double ellipses, which represent multivalued attributes 6. Dashed ellipses, which denote derived attributes 7. Double links, which indicate total participation of an entity i

n a relationship set


many to many

borrower loancustomer 1 M

borrower loancustomer M 1

borrower loancustomer M M

borrower loan

amountcustomer-city

loan-number

customer

customer-name

customer-id

customer-street

one to many

many to one

one to one

depositor account

balancecustomer-city

account-number

customer

customer-name

customer-id

customer-street


access-date

We have the access-date descriptive attribute attached to the relationship set depositor to specify the most recent date on which a customer accessed that account.

Figure 2.10

E-R diagram with an attribute attached to a relationship set


name

date-of-birth

street-name

customer

first-name

customer-id

street

last-name

middle-initial

address

city

state

zip-code

phone-number

street-number apartment-number

age

multivalued attributes

derived attributes

composite

attributes

Example:


works-for

telephone-number

employee

employee-name

employee-id

manager

worker

Figure2.12 E-R diagram with role indicators

role


works-on branch

branch-citycity

branch-name

employee

employee-name

employee-id

street

assets

job

leveltitle

Figure2.13 E-R diagram with a ternary relationship


borrower loan

amountcustomer-city

loan-number

customer

customer-name

customer-id

customer-street

total participation

Figure2.14 Total participation of an entity set in a relationship set

borrower loan

amountcustomer-city

loan-number

customer

customer-name

customer-id

customer-street


0..* 1..1

Figure2.15 Cardinality limits on relationship sets

l..h: l minimum cardinality h maximum cardinality

A minimum value of 1 : total participation

A maximum value of 1 : the entity participates in at most one relationship

A maximum value * : no limit 1..* =total participation

2.6 Weak Entity Sets

An entity set may not have sufficient attributes to form a primary key. Such an entity set is termed a weak entity set.

primary key of payment = loan-number + payment-numberprimary key of loan discriminator

payment-number

paymentloan-paymentloan

loan-number amount

M1

payment-date payment-amount

discriminator

doubly outlined diamond

doubly outlined box

weak entity set

strong entity set

total participation

Example:(E-R) partly

A manufacturing company will wish to record information about :the projects it has on hand;the parts used in those projects;the suppliers who supply those parts;the employees who work on those projects;the departments which employees belong to; the employee’s dependent .

The employee comprise those information: the number of the employee, employee’s name, (include first name ... ) and the salary.

The supplier comprise those information: the number of the supplier, the supplier’s name, supplier’s address(include status and city)

A employee(manager,worker) must belong to a department and may take part in more than one projects

A manager is responsible for more than one projects but a project should assign to one manger

A supplier provide a certain quantity parts to projects.

M

MPROJ-WORK

QTY

PROJ-MANAGER

PROJECTSUPP

PART_PROJ

SUPP_PART

PART

PART_STRUCTURE

QTY

SUPPLIER

CITYSTATUSSNAMES#

M M

M

M

M

M

M M

M

1

DEPENDENT

EMP_DEP

DEPT-EMP

EMPLOYEE

DEPARTMENT

SALARY

EMP#

ENAME

FIRST MI LAST

M

1

1

total participation

many to many

one to many

weak entity set

composite

attributes

2.7 Extended E-R Features

1. Specialization An entity set may include subgroupings of entities that are distinct in some way from othe

r entities in the set. The process of designating subgroupings within an entity set is specialization.

Specialization emphasizes differences among entities within the set

2. Generalization This commonality can be expressed by generalization, which is a containment relationshi

p that exists between a higher-level entity set and one or more lower -level entity sets. generalization emphasizes commonalties among entity sets

ISA

employee is a person

hours-worked

officer

hours-worked

customer

citystreetname

employee

person

credit-ratingsalary

office-number

station-number

ISA

ISA

teller secretary

specialization

generalization

officer is a employee

Figure2.17 Specialization and generalization


Total(double line) partial(line)

disjoint (d) overlapping(o)

employee

ISA

customer

person

savings-account

ISA

checking-account

account

disjointoverlapping

partial participation total participation


3. Attribute Inheritance A crucial property of the higher-and lower-level entities created b

y specialization and generalization is attribute inheritance. The attributes of the higher-level entity sets are said to be inherited by the lower-level entity sets.

If an entity set is a lower-level entity set in more than one ISA relationship, the resulting structure is said to be a lattice.

A higher-level entity set with attributes and relationships that apply to all of its lower-level entity sets.

Lower-level entity sets with distinctive features that apply only within a particular lower-level entity set.


4. Aggregation One limitation of the E-R modal is that it is not possible to express relationships among relationships.

works-on branchemployee

job

manages

manager

redundant relationships

Figure2.18 E-R diagram with redundant relationships

works-on branchemployee

job

manages

manager


Aggregation is an abstraction through which relationship are treated as higher-level entities.

aggregationhigher-level

entities

Figure2.19 E-R diagram with aggregation

2.8 Reduction of an E-R Schema to Tables

A database that conforms to an E-R database schema can be represented by a collection of tables. 1. Tabular Representation of Strong Entity Sets

In general, if we have a table of n columns. We denote the Cartesian product of D1,D2……Dn by D1×D2 ×…… ×Dn-1 ×Dn.

1. Tabular Representation of Strong Entity Sets

telephone-number

employee

employee-name

employee-id employee-name

employee-id

telephone-number

employee


2. Tabular Representation of Weak Entity Sets

Let A be a weak entity set with attributes a1,a2……an. Let B be the strong entity set on which A is dependent. Let the primary key of B consist of attributes b1,b2……bn. {a1,a2……am} {b∪ 1,b2……bn}

payment-number


loan-number amount

M1


loan-number payment-number

payment-date

payment-amount

payment

3. Tabular Representation of Relationship Sets


depositor account

balancecustomer-city

account-number

customer

customer-name

customer-id

customer-street

access-date

customer-id account-number access-datedepositor


⑴ Redundancy of Tables In general, the table for the relationship set linking a weak entity set to its corresponding strong entity set is redundant and does not need to be present in a tabular representation of an E-R diagram.

payment-number


loan-number amount

M1


loan-number

payment-number

payment-date

payment-amount

loan-number

payment-number

redundancy

loan-payment payment

⑵ Combination of Tables (1 to many)


account-branch branch

assetsbranch-name

account

account-number branch-citybalance

account-number balance account-number branch-name

account-number balance branch-name

accountaccount-branch

combination


4. Composite Attributes

customer-street

customer

customer-city

customer-id

customer-address

customer-name

customer-id customer-name customer-street customer-city

customer


5. Multivalued Attributes

New tables should be created for multivalued attributes.

For a multivalued attributes M, we create a table T with a column C that corresponds to M and columns corresponding to the primary key of the entity set or relationship set of which M is an attribute.

employee-id

employee

…

dependent-name…

employee-id d-name

dependent-name


6. Tabular Representation of Generalization

account-number

savings-account

overdraft-amount

account

interest-rate

balanceISA

checking-account


6. Tabular Representation of Generalization 1、 Create a table for the higher-level entity set. For each lower-

level entity set, create a table that includes a column for each of the attributes of that entity set plus a column for each attribute of the primary key of the higher-level entity set.

account-number balance account

account-number interest-rate

savings-account checking-account

account-number overdraft-amount


2、 If the generalization is disjoint and complete then we should not create a table for the higher-level entity set. Instead, for each lower-level entity set, create a table that includes a column for each of the attributes of that entity set plus a column for each attribute of the higher-level entity set.

account-number balance interest-ratesavings-account

account-number balance overdraft -amountchecking-account


7. Tabular Representation of Aggregation

borrower loancustomer

loan-numbercustomer-name

loan-officer

employee

employee-numbercustomer-name

loan-number

employee-number

loan-officer

Exercises:

P72: 2.1 2.3 2.5 2.6 2.9 2.12

Documents

Chapter 2 Entity-Relationship Model 2.1 IntroductionIntroduction 2.2 Basic conceptsBasic concepts 2.3 mapping constrainsmapping constrains 2.4 keyskeys