Chapter2

2. Entity Relationship(ER) Model DATABASE MANAGEMENT SYSTEMS

THE RELATİON MODEL ,[object Object],[object Object],[object Object],[object Object]

BASİC CONCEPT ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

[object Object],[object Object],[object Object],[object Object],BASİC CONCEPT

AN ENTİTY SET İS A SET OF ENTİTİES OF THE SAME TYPE ,[object Object],Employee Entity Type : e 1 e 2 e 3 Entity set: … The actual employees A general specification

ATTRİBUTES ,[object Object],[object Object],[object Object],[object Object]

TYPES OF ATTRİBUTES ,[object Object],Employee EmpNo Name Address

CONPOSİTE ATTRİBUTE:CONSİST OF SEVERAL COMPONENTS Country Employee Address Street City EmpNo Name

MULTİVALUE ATTRİBUTE :CONTAİNS MORE THAN ONE VALUE Employee Phone Email

DERİVED ATTRİBUTE:COMPUTED FROM OTHER ATTRİBUTES Employee Age Bonus

KEY ATTRİBUTES ,[object Object],ERD tabular Employee EmpNo Name EmpNo Name . . . 123456 John Wong . . . 456789 . . . 146777 . . . John Wong Mary Cheung

KEY ATTRİBUTES ,[object Object],Employee Name Address

KEY ATTRİBUTES ,[object Object],[object Object],[object Object],In many cases, a key is artificially introduced (e.g., EmpNo) to make applications more efficient. Employee EmpNo Name Address

RELATİONSHİP ,[object Object],[object Object],[object Object]

[object Object],[object Object],Question: Could Amount be an attribute of Customer? Or an attribute of Account? What does Amount mean? How many values you want to keep? Customer depositor Account Amount CusNo Name AccNo Name

REPRESENTATİON OF RELATİONSHİP ,[object Object],Depositor Note: this is NOT an ERD The amount in each deposit. CustomerNo A123456 B456789 B456789 AccountNo A-101 A-201 A-302 Amount 500 900 700 A123456 B456789 A-101 A-201 A-202 500 900 700 ,[object Object]

TRY AN ALTERNATİVE ,[object Object],AccountNo A-101 A-201 A-302 Name Current Saving Current Amount 500 900 700 ,[object Object],[object Object]

CARDİNALİTY OF RELATİONSHİPS ,[object Object],[object Object],Customer Loan Borrows ,[object Object]

1 : N Customer Loan One entity of type X can be associated with many entities of type Y . One entity of type Y can be associated with, at most, one entity of type X . Customer Borrow Loan ,[object Object],[object Object]

N : M One entity of type X can be associated with many entities of type Y . One entity of type Y can be associated with many entities of type X . Customer Borrow loan ,[object Object],[object Object]

NOTES ,[object Object],[object Object],[object Object],[object Object],[object Object]

DEGREES OF A RELATİONSHİPS SET ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Customer Loan Borrow - Binary Customer Loan Borrow Branch - Ternary

RECURSİVE RELATİONSHİPS ,[object Object],[object Object],[object Object],[object Object],manager worker Employee work-for Employee manager worker Employee work-for

TABULAR REPRESENTATİON OF RECURSİVE RELATİONSHİPS ,[object Object],[object Object],[object Object],ManagerNo WorkerNo A1234 A6543 A1234 A8734 EmployeeNo EmployeeNo A1234 A8734 A1234 A6543

OPTİONAL VERSUS MANDATORY ,[object Object],[object Object],[object Object],[object Object],[object Object]

OPTİONAL VERSUS MANDATORY Neither one is preferable to the other. The proper one to choose depends on the business situation. Mandatory sub-typing is represented by creating a double line from the super-type ( person in the following ER diagram) to the circle. Optional sub-typing is represented by leaving a single line from the super-type to the circle.

AGGREGATİON OF ENTİTY TYPES ,[object Object]

AGGREGATİON OF ENTİTY TYPES Consider the relationship between a car and its engine and body. The engine and body are both part of the car. The relationship is represented as follows in an ER diagram.

PARALLEL RELATİONSHİPS ,[object Object]

PARALLEL RELATİONSHİPS A person pays for zero or more insurance policies. An insurance policy is paid for by exactly one person. A person is insured by zero or more insurance policies. An insurance policy insures one or more persons.

EXİSTENCE DEPENDENCE ,[object Object],Customer Loan loan borrow CusNo LoanNo A loan cannot exist if there is no borrower.

WEAK ENTİTİES ,[object Object],[object Object]

LoanNo Amount Date_pay PaymentNo Payment ,[object Object],[object Object],[object Object],[object Object],Loan payment Question: Why not combine loan and payment into one entity type? Amount Loan

WEAK ENTİTY VS EXİSTENCE CONSTRAİNTS ,[object Object],[object Object]

ANOTHER EXAMPLE OF WEAK ENTİTY TYPE ,[object Object],[object Object],EmpNo Name Dependent Emp_Dep Age Employee

TERNARY RELATİONSHİPS Customer Loan Borrow Branch A customer borrows a loan from a branch. Customer Loan Borrow Branch Issue A customer borrows a loan. A loan is issued from a branch.

WHAT ARE THE DİFFERENCES? A customer borrows a loan from a branch. A customer borrows a loan. A loan is issued from a branch.

[object Object],[object Object],[object Object],Customer Loan Borrow Branch Issue Cus_Br

WHY? ,[object Object],[object Object],[object Object],A12345 B56789 A12345 A12345 L-001

BİNARY RELATİONSHİPS TO TERNARY? ,[object Object],[object Object],Customer Loan Borrow Branch Issue Buy_stock

A CASE STUDY A primary school student writes a composition about a picnic: Today is Sep 9, the weather is fine. My classmates, John, Mary and I go to a picnic in Sai Kung. Our teacher is Ms Wong Initial Design: Picnic weather destination date Students Name Teacher Name

QUESTİONS ? ,[object Object],[object Object],[object Object],[object Object]

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],My solution Question: How to record number of students in a picnic? Student StudentNo Name weather date destination Picnic goes leading Teacher Name

E-R DESİGN DECİSİONS ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

E-R DİAGRAM FOR COMPANY DATABASE EMPLOYEE WORKS_FOR MANAGES CONTROLS Startdate DEPARTMENT WORKS_ON PROJECT Hours DEPENDENTS_OF DEPENDENT SUPERVISION supervisee supervisor Fname Minit Lname Name Sex Address Salary Ssn Bdate Number Of Employees Locations Can you translate it back into English? Name Number Name Number Location Relationship Birthdate Sex Name

LİMİTATİONS OF ER MODEL ,[object Object],[object Object],[object Object],[object Object]

REDUCTİON OF AN E-R SCHEMA TO TABLES ,[object Object],[object Object],[object Object]

REPRESENTİNG ENTİTY SETS AS TABLES ,[object Object],[object Object],Composite key payment loan-no payment-date payment-no payment-amount L-17 L-15 L-23 5 22 11 10 May 1999 23 May 1999 17 May 1999 50 300 75 customer customer-name customer-street customer-id Jones Hayes Smith 321-12-3123 677-89-9011 019-28-3746 Main Main North customer-city Harrison Rye Harrison

REPRESENTİNG RELATİONSHİP SETS AS TABLES depositor ,[object Object],customer borrow loan loan-no cust-no share% date cust-name

[object Object],[object Object],customer cust-no cust-name A12345 B56789 Peter Wong Mary Cheung loan loan-no L-001 L-002 date Sep 2000 Aug 2001 customer borrow loan cust-no cust-name date loan-no cust-no A12345 B56789 indicates who borrowed the loan

WEAK ENTİTİES ,[object Object],Already indicates the 1:N relationship between loan-no and payment-no payment loan-no payment-date payment-no payment-amount L-17 L-17 L-17 5 7 6 10 May 1999 23 May 1999 17 May 1999 50 300 75

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