Transcript Entity Relationship (E-R) Modeling

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Chapter 4
Entity Relationship (E-R) Modeling
Database Systems: Design, Implementation, and Management
4th Edition
Peter Rob & Carlos Coronel
Developing an E-R Diagram
 The process of database design is an iterative
rather than a linear or sequential process.
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 It usually begins with a general narrative of the
organization’s operations and procedures.
 The basic E-R model is graphically depicted and
presented for review.
 The process is repeated until the end users and
designers agree that the E-R diagram is a fair
representation of the organization’s activities and
functions.
Developing an E-R Diagram
 Tiny College Database (1)
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
Tiny College (TC) is divided into several schools.
Each school is administered by a dean. A 1:1
relationship exists between DEAN and SCHOOL.

Each dean is a member of a group of
administrators (ADMINISTRATOR). Deans also
hold professorial rank and may teach a class
(PROFESSOR). Administrators and professors
are also Employees. (Figure 4.38)
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Developing an E-R Diagram
 Tiny College Database (2)
Each school is composed of several departments.
 The smallest number of departments operated by a
school is one, and the largest number of departments
is indeterminate (N).
 Each department belongs to only a single school.

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Figure 4.40 The First Tiny College ERD Segment
Developing an E-R Diagram
 Tiny College Database (3)

Each department offers several courses.
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Figure 4.41
The Second Tiny College ERD Segment
Developing an E-R Diagram
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 Tiny College Database (4)
 A department may offer several sections (classes) of the
same course.
 A 1:M relationship exists between COURSE and CLASS.
 CLASS is optional to COURSE
Figure 4.42 The Third Tiny College ERD Segment
Developing an E-R Diagram
 Tiny College Database (5)

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
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Each department has many professors assigned to it.
One of those professors chairs the department. Only
one of the professors can chair the department.
DEPARTMENT is optional to PROFESSOR in the
“chairs” relationship.
Figure 4.43
The Fourth Tiny College ERD Segment
Developing an E-R Diagram
 Tiny College Database (6)
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
Each professor may teach up to four classes,
each one a section of a course.
A professor may also be on a research contract
and teach no classes.
Figure 4.44 The Fifth Tiny College ERD Segment
Developing an E-R Diagram
 Tiny College Database (7)

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

A student may enroll in several classes, but (s)he
takes each class only once during any given enrollment
period.
Each student may enroll in up to six classes and each
class may have up to 35 students in it.
STUDENT is optional to CLASS.
Figure 4.45
The Sixth Tiny College ERD Segment
Developing an E-R Diagram
 Tiny College Database (8)
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
Each department has several students whose major is
offered by that department.
Each student has only a single major and associated
with a single department.
Figure 4.46
The Seventh Tiny College ERD Segment
Developing an E-R Diagram
 Tiny College Database (9)

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
Each student has an advisor in his or her department;
each advisor counsels several students.
An advisor is also a professor, but not all professors
advise students.
Figure 4.47
The Eighth Tiny College ERD Segment
Developing an E-R Diagram
Entities for the Tiny College Database
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 SCHOOL
 COURSE
 DEPARMENT
 CLASS
 EMPLOYEE
 ENROLL (Bridge between
STUDENT and CLASS)
 PROFESSOR
 STUDENT
Components of the E-R Model
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Table 4.2
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Figure 4.48
Developing an E-R Diagram
 Converting an E-R Model into a Database Structure
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
A painter might paint many paintings. The
cardinality is (1,N) in the relationship between
PAINTER and PAINTING.

Each painting is painted by one (and only one)
painter.
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A painting might (or might not) be exhibited in a
gallery; i.e., the GALLERY is optional to
PAINTING.
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Figure 4.49
Developing an E-R Diagram
 Summary of Table Structures and Special
Requirements for the ARTIST database
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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
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Table 4.3
Developing an E-R Diagram
SQL Commands to Create the PAINTER Table
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CREATE TABLE PAINTER (
PTR_NUM
CHAR(4)
PRT_LASTNAME CHAR(15)
PTR_FIRSTNAMECHAR(15),
PTR_INITIAL
CHAR(1),
PTR_AREACODE CHAR(3),
PTR_PHONE
CHAR(8),
PRIMARY KEY(PTR_NUM));
NOT NULL UNIQUE,
NOT NULL,
Developing an E-R Diagram
SQL Commands to Create the GALLERY Table
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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));
Developing an E-R Diagram
SQL Commands to Create the PAINTING Table
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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 RESTRICT
ON UPDATE CASCADE,
FOREIGN KEY(GAL_NUM) REFERENCES GALLERY
ON DELETE RESTRICT
ON UPDATE CASCADE);
Developing an E-R Diagram
 General Rules Governing Relationships
among Tables
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1.All primary keys must be defined as NOT NULL.
2.Define all foreign keys to conform to the
following requirements for binary relationships.

1:M Relationship

Weak Entity
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M:N Relationship

1:1 Relationship
Developing an E-R Diagram
 1:M Relationships
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
Create the foreign key by putting the primary 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 are
OPTIONAL
NULL
ALLOWED
SET NULL
CASCADE
If one side is
OPTIONAL and
the other
MANDATORY
NULL
ALLOWED
SET NULL
or
RESTRICT
CASCADE
Developing an E-R Diagram
 Weak Entity

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
Put the key of the parent table (strong entity) in the
weak entity.
The weak entity relationship conforms to the same
rules as the 1:M relationship, except foreign 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.
Developing an E-R Diagram
 1:1 Relationships
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If both entities are in mandatory participation
in the relationship and they do not participate in
other relationships, it is most likely that the
two entities should be part of the same entity.
Developing an E-R Diagram
 CASE 1: M:N, Both Sides MANDATORY
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Figure 4.50 Entity Relationships, M:N, Both Sides Mandatory
Developing an E-R Diagram
 CASE 2: M:N, Both Sides OPTIONAL
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Figure 4.51 Entity Relationships, M:N, Both Sides Optional
Developing an E-R Diagram
 CASE 3: M:N, One Side OPTIONAL
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Figure 4.52
Entity Relationships, M:N, One Side Optional
Developing an E-R Diagram
 CASE 4: 1:M, Both Sides MANDATORY
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Figure 4.53 Entity Relationships, 1:M, Both Sides Mandatory
Developing an E-R Diagram
 CASE 5: 1:M, Both Sides OPTIONAL
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Figure 4.54 Entity Relationships, 1:M, Both Sides Optional
Developing an E-R Diagram
 CASE 6: 1:M, Many Side OPTIONAL, One Side
MANDATORY
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Figure 4.55 Entity Relationships, 1:M, Many Side Optional, One Side Mandatory
Developing an E-R Diagram
 CASE 7: 1:M, One Side OPTIONAL, One Side
MANDATORY
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Figure 4.56 Entity Relationships, 1:M, One Side Optional, Many Side Mandatory
Developing an E-R Diagram
 CASE 8: 1:1, Both Sides MANDATORY
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Figure 4.57
Entity Relationships, 1:1, Both Sides Mandatory
Developing an E-R Diagram
 CASE 9: 1:1, Both Sides OPTIONAL
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Figure 4.58 Entity Relationships, 1:1, Both Sides Optional
Developing an E-R Diagram
 CASE 10: 1:1, One Side OPTIONAL, One Side
MANDATORY
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Figure 4.59 Entity Relationships, 1:1, One Side Optional, One Side Mandatory
Developing an E-R Diagram
 CASE 11: Weak Entity (Foreign key located in weak
entity)
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Figure 4.60 Entity Relationships, Weak Entity
Developing an E-R Diagram
 CASE 12: Multivalued Attributes
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Figure 4.61 Entity Relationships, Multivalued Attributes
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The Chen Representation of the Invoicing Problem
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Figure 4.63
The Crow’s Foot Representation of the Invoicing Problem
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Figure 4.64
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Figure 4.65 The Rein85 Representation of the Invoicing Problem
The IDEF1X Representation of the Invoicing Problem
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Figure 4.66
The Challenge of Database Design:
Conflicting Goals
 Conflicting Goals
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Design standards (design elegance)
Processing speed
Information requirements
 Design Considerations
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Logical requirements and design conventions
End user requirements; e.g., performance,
security, shared access, data integrity
Processing requirements
Operational requirements
Documentation