e r model chapter 5

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Chapter 5Entity Relationship (E-R)

Modeling



Developing an E-RDiagram

• The process of database design is an iterativerather than a linear or sequential process.

• It usually begins with a general narrative of the

organization’s operations and procedures.

• The basic E-R model is graphically depicted andpresented for review.

• The process is repeated until the end users anddesigners agree that the E-R diagram is a fairrepresentation of the organization’s activities andfunctions.



• Tiny College Database (1)

– Tiny College (TC) is divided into severalschools. Each school is administered by adean. A 1:1 relationship exists betweenDEAN and SCHOOL.

– Each dean is a member of a group of

administrators (ADMINISTRATOR). Deansalso hold professorial rank and may teach aclass (PROFESSOR). Administrators andprofessors are also Employees. (Figure 4.38)

Developing an E-R

Diagram



Developing an E-R

Diagram• Tiny College Database (2)

– Each school is composed of severaldepartments.

– The smallest number of departmentsoperated by a school is one, and the largestnumber of departments is indeterminate (N).

– Each department belongs to only a single

school.

Figure 4.40 The First Tiny College ERD Segment



• Tiny College Database (3)– Each department offers several

courses.

Developing an E-R

Diagram

Figure 4.41 The Second Tiny College ERD Segment




– A department may offer several sections(classes) of the same course.

– A 1:M relationship exists between COURSEand CLASS.

– CLASS is optional to COURSE


Figure 4.42 The Third Tiny College ERD Segment



• Tiny College Database (5)– Each department has many professors

assigned to it.

– One of those professors chairs the

department. Only one of the professors canchair the department.

– DEPARTMENT is optional to PROFESSOR inthe “chairs” relationship.


Figure 4.43 The Fourth Tiny College ERD Segment




– Each professor may teach up to fourclasses, each one a section of a course.

– A professor may also be on a researchcontract and teach no classes.


Figure 4.44 The Fifth Tiny College ERD Segment



• Tiny College Database (7)– A student may enroll in several classes, but

(s)he takes each class only once during anygiven enrollment period.

– Each student may enroll in up to six classesand each class may have up to 35 studentsin it.

– STUDENT is optional to CLASS.


Figure 4.45 The Sixth Tiny College ERD Segment




– Each department has several students whosemajor is offered by that department.

– Each student has only a single major andassociated with a single department.


Figure 4.46 The Seventh Tiny College ERD Segment



• Tiny College Database (9)– Each student has an advisor in his or her

department; each advisor counsels severalstudents.

– An advisor is also a professor, but not allprofessors advise students.


Figure 4.47 The Eighth Tiny College ERD Segment



Entities for the Tiny College Database

• SCHOOL

• DEPARMENT

• EMPLOYEE

• PROFESSOR

• COURSE• CLASS

• ENROLL (Bridge between

STUDENT and CLASS)• STUDENT




Components of the E-R Model

Table 4.2



Figure 4.48



• Converting an E-R Model into a DatabaseStructure

– A painter might paint many paintings. Thecardinality is (1,N) in the relationshipbetween PAINTER and PAINTING.

– Each painting is painted by one (and only

one) painter.– A painting might (or might not) be exhibited

in a gallery; i.e., the GALLERY is optional toPAINTING.




Figure 4.49



• Summary of Table Structures and SpecialRequirements for the ARTIST database

PAINTER(PRT_NUM, PRT_LASTNAME,PRT_FIRSTNAME, PRT_INITIAL,

PTR_AREACODE, PRT_PHONE)

GALLERY(GAL_NUM, GAL_OWNER,

GAL_AREACODE, GAL_PHONE, GAL_RATE)PAINTING(PNTG_NUM, PNTG_TITLE,

PNTG_PRICE, PTR_NUM, GAL_NUM)




A Data Dictionary for the ARTIST Database

Table 4.3

l E



SQL Commands to Create the PAINTER Table

CREATE TABLE PAINTER (

PTR_NUM CHAR(4) NOT NULL UNIQUE,

PRT_LASTNAME CHAR(15) NOT

NULL,

PTR_FIRSTNAME CHAR(15),PTR_INITIAL CHAR(1),

PTR_AREACODE CHAR(3),

PTR_PHONE CHAR(8),

PRIMARY KEY(PTR_NUM));




SQL Commands to Create the GALLERY Table

CREATE TABLE GALLERY (

GAL_NUM CHAR(4) NOT NULL UNIQUE,

GAL_OWNER CHAR(35),

GAL_AREACODE CHAR(3) NOT NULL,

GAL_PHONE CHAR(8) NOT NULL,GAL_RATE NUMBER(4,2),

PRIMARY KEY(GAL_NUM));


D l i E R



SQL Commands to Create the PAINTINGTable

CREATE TABLE PAINTING (

PNTG_NUM CHAR(4) NOT NULL UNIQUE,

PNTG_TITLE CHAR(35),

PNTG_PRICE NUMBER(9,2),PTR_NUM CHAR(4) NOT NULL,

GAL_NUM CHAR(4),

PRIMARY KEY(PNTG_NUM)

FOREIGN KEY(PTR_NUM) RERERENCES PAINTER

ON DELETE RESTRICTON UPDATE CASCADE,

FOREIGN KEY(GAL_NUM) REFERENCES GALLERY

ON DELETE RESTRICT

ON UPDATE CASCADE);


D l E R



• General Rules Governing Relationships amongTables

1. All primary keys must be defined as NOTNULL.

2. Define all foreign keys to conform to thefollowing requirements for binaryrelationships.

• 1:M Relationship• Weak Entity

• M:N Relationship

• 1:1 Relationship


l E



• 1:M Relationships

– Create the foreign key by putting theprimary key of the “one” (parent) in the

table of the “many” (dependent). – Foreign Key Rules:

Null On Delete On Update

If both sides are

MANDATORY

NOT NULL RESTRICT CASCADE

If both sides areOPTIONAL

NULLALLOWED

SET NULL CASCADE

If one side isOPTIONAL and

the other

MANDATORY

NULLALLOWED

SET NULLor

RESTRICT

CASCADE


D l i E R



• Weak Entity

– Put the key of the parent table (strongentity) in the weak entity.

– The weak entity relationship conforms to thesame rules as the 1:M relationship, exceptforeign key restrictions:

NOT NULL

ON DELETE CASCADE

ON UPDATE CASCADE • M:N Relationship

– Convert the M:N relationship to a composite(bridge) entity consisting of (at least) the

parent tables’ primary keys.




D l E R



• CASE 1: M:N, Both Sides MANDATORY


Figure 4.50 Entity Relationships, M:N, Both Sides Mandatory



• CASE 2: M:N, Both Sides OPTIONAL

Developing an E-R

Diagram

Figure 4.51 Entity Relationships, M:N, Both Sides Optional



• CASE 4: 1:M, Both Sides MANDATORY


Figure 4.53 Entity Relationships, 1:M, Both Sides Mandatory



• CASE 5: 1:M, Both Sides OPTIONAL


Figure 4.54 Entity Relationships, 1:M, Both Sides Optional



• CASE 6: 1:M, Many Side OPTIONAL,One Side MANDATORY

Developing an E-R

Diagram

Figure 4.55 Entity Relationships, 1:M, Many Side Optional, One Side Mandatory



• CASE 7: 1:M, One Side OPTIONAL,One Side MANDATORY

Developing an E-R

Diagram

Figure 4.56 Entity Relationships, 1:M, One Side Optional, Many Side Mandatory



• CASE 8: 1:1, Both SidesMANDATORY

Developing an E-R

Diagram

Figure 4.57 Entity Relationships, 1:1, Both Sides Mandatory



• CASE 9: 1:1, Both Sides OPTIONAL

Developing an E-R

Diagram

Figure 4.58 Entity Relationships, 1:1, Both Sides Optional



• CASE 10: 1:1, One Side OPTIONAL,One Side MANDATORY

Developing an E-R

Diagram

Figure 4.59 Entity Relationships, 1:1, One Side Optional, One Side Mandatory

l E



• CASE 11: Weak Entity (Foreign keylocated in weak entity)

Developing an E-R

Diagram

Figure 4.60 Entity Relationships, Weak Entity

l E



• CASE 12: Multivalued Attributes

Developing an E-R

Diagram

Figure 4.61 Entity Relationships, Multivalued Attributes



The Chen Representation of the Invoicing Problem

Figure 4.63



The Crow’s Foot Representation of the Invoicing Problem

Figure 4.64



Figure 4.65 The Rein85 Representation of the Invoicing Problem



The IDEF1X Representation of the Invoicing Problem

Figure 4.66

Th Ch ll f D t b D i



The Challenge of Database Design:Conflicting Goals

• Conflicting Goals

– Design standards (design elegance)

– Processing speed

– Information requirements

• Design Considerations– Logical requirements and design

conventions

– End user requirements; e.g.,

performance, security, shared access,data integrity

– Processing requirements

– Operational requirements

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