lecture6 dbms

7/27/2019 Lecture6 DBMS

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Entity relation Diagram


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IntroductionStrong & Weak Entity Sets An entity set that has a primary key is termed as a Strong entity set. If an entity set may

not have sufficient attributes to form a primary key. Then these entity sets are termed as a Weak entity set. For example Employee entity set has three attributes, Employee_ID, Department andProject and any one of them can act as a key attribute or primary key; further we haveanother entity set Dependent having three attributes as Name, Age and Sex. In Dependententity set there is no such attribute, which can act as a key attribute so this, is known as

weak entity set and Employee is called as Strong entity set. The primary key of the strong entity set forms the primary key of a weak entity set on which the weak entity set isexistence dependent.

Have Employee

Employee_ID

Department

Project

Dependent

Age Sex

Name


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1.6

Attributes Can be Attached to a Relationship Set

If a relationship set has also some attributes associated with

it, then we link these attributes to that relationship set.

Customer

Customer Name

Social SecurityNo

Customer Street

Customer City

Borrow er

Balance

Account

Account_No

Access_Date


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Role Indicators A relationship might associate several entities for the same underlying entity set. Then an additional role indicator is used in the E-R diagram to

distinguish the similar entities. The function that an entity plays in a relationship is called its Role . Forexample, the relationship W orks-As might be ordered pairs of E mployee (first is Team Member, second is Project Leader).

Project Leader

WorksA

s

Employee

Telephone

Emp_ID Emp_Name

Team Member


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Non-Binary (Ternary) Relationships Diagram specifies that a customer may have several loans, and that loanmay belong to several different customers.

Arrow pointing to branch indicates that each customer-loan pair isassociated with a specific bank branch.

Amount

Loan

Loan_No

CLB

Branch

Branch Name

Branch City

Phone

Customer

Customer Name

Social SecurityNo

Customer Street

Customer City


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Enhanced ER (EER) Model The ER modeling concepts discussed earlier are sufficient for representing many database schemas for traditional database applications, which mainly include data-processing applications in business and industry.

The newer applications of database technology include databases forengineering design and manufacturing (CAD/CAM), telecommunications,

images and graphics, multimedia, data mining, data warehousing,geographic information systems (GIS), and databases for indexing the World Wide Web among many other applications.

To represent these requirements as accurately and clearly as possible, theER model can be enhanced to include these concepts, leading to theEnhanced-ER or EER model.


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The EER model includes all the modeling concepts of the ER modeland the concepts of Subclass and Superclass and the relatedconcepts of Specialization and Generalization .

One more concept EER model is Category , which is used torepresent a collection of objects. It is the union of objects of differententity types.


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SpecializationAn entity set may include subgroupings of entities that are distinct in someway from other entities in the set. A subset of entities within an entity set may

have attributes that are not shared by all the entities in the entity set.Ex. An entity set person may be classified as

Customer Employee

Each person types is described by a set of attributes that includes all theattributes of entity set person plus some additional attributes.

The process of designating sub groupings within an entity set is calledspecial izat ion.

A person can be a employee, a customer or both.


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Person

Employee Customer

Person_id name street city

salary Credit_rating

officer teller secretary

Office_number

Station_number

Hours_worked

Hours_worked

Fig. Specialization and generalization


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Generalization:

A generalization is simply inversion of specialization. In other words a Generalization is acontainment relationship that exist between a higher level entity set to one or more lower

level entity set.To create a generalization the attributes must be given a common name and representedfrom the higher level to lower level.

Higher and lower level entity sets are also called as superc lass and subc l a s s .

In nutshell, the specialization process allows us to do the following 1. Define a set of subclasses of an entity type. 2. Establish additional specific attributes with each subclass. 3. Establish additional specific relationship types between eachsubclass and other entity types or other subclasses.


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Aggregation:

the limitation of E-R model is that it cannot express relationships amongother relationships.

Ex. We want to record managers for tasks performed by an employee at abranch (I.e. we want to record managers for(employee, branch , job)combinations.

Assume there is an entity set manager.create a quaternary relationshipmanages

works_on

manages

branchemployee

manager

job


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Aggregation is an abstraction through which relationships are treated as higher levelentities.

Here the relationship set works_on (relating entity set employee, branch and job) act ashigher level entity set called works_on. Now we can create the binary relationship manages

between works_on and manager to represent who manages the task.

works_on

manages

branchemployee

manager

job

E-R diagram with aggregation


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Reduction of an E-R schema to Tables: -

Tabular representation of strong entity sets : - Let E be a strong entity setwith descriptive attributes a

1, a

2, a

3, --------, a

n. We represent this entity by a table

called E with n distinct columns, each of which corresponds to one of the attributesof E. Each row in this table corresponds to one entity of the entity set E.

Ex. the loan entity set has two attributes, loan_number andamount.

Loan_number Amount

L-17L-18L-23L-25L-27L-11L-19

1000200035004000300015002500

Loan Table


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Tabular Representation of weak entity set: - Let A be a weak entity set with attributes a 1,a2, a 3, ----, a m. Let B be the strong entity set on which A is dependent. Let the primary key of B consist of attributes b 1, b2, b3, ----, b n. We represent the entity set A by a table called A withone column for each attribute of the set

{a1, a2, a3, -----,a m} U {b 1, b2, b3, ----, b n}

Ex. payment entity set has three attributes payment_number, payment_date and payment_amount. The primary key of the loan entity set on which payment is dependent islaon_number.

Loan_number Payment_number Payment_date Payment_amount

L-17L-18L-23L-25L-27L-11L-19

57

1215131110

10-05-200015-06-200010-04-199915-12-199905-04-200012-11-199829-10-2001

500150030001500100020001500


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Assignment

Q. Explain Strong and Weak Entity Sets.

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