Transcript Entity Relationship (ER) Modeling

Chapter 5
Entity Relationship (E-R)
Modeling
Developing an E-R
Diagram
• The process of database design is an iterative
rather 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 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)
– 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)
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.
Figure 4.40 The First Tiny College ERD Segment
Developing an E-R
Diagram
• Tiny College Database (3)
– Each department offers several
courses.
Figure 4.41 The Second Tiny College ERD Segment
Developing an E-R
Diagram
• 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)
– 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)
– 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)
– 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)
– 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)
– 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
• SCHOOL
• COURSE
• DEPARMENT
• CLASS
• EMPLOYEE
• ENROLL (Bridge between
STUDENT and CLASS)
• PROFESSOR
• STUDENT
Components of the E-R Model
Table 4.2
Figure 4.48
Developing an E-R
Diagram
• Converting an E-R Model into a Database
Structure
– 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.
– A painting might (or might not) be exhibited
in a gallery; i.e., the GALLERY is optional to
PAINTING.
Figure 4.49
Developing an E-R
Diagram
• Summary of Table Structures and Special
Requirements 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
Developing an E-R
Diagram
SQL Commands to Create the PAINTER Table
CREATE TABLE PAINTER (
PTR_NUM
CHAR(4) NOT NULL
UNIQUE,
PRT_LASTNAME
CHAR(15)
NULL,
PTR_FIRSTNAME
CHAR(15),
PTR_INITIAL
CHAR(1),
PTR_AREACODE
CHAR(3),
PTR_PHONE CHAR(8),
PRIMARY KEY(PTR_NUM));
NOT
Developing an E-R
Diagram
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));
Developing an E-R
Diagram
SQL Commands to Create the PAINTING
Table
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
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
• M:N Relationship
• 1:1 Relationship
Developing an E-R
Diagram
• 1:M Relationships
– 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
– 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
– 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
Figure 4.50 Entity Relationships, M:N, Both Sides Mandatory
Developing an E-R
Diagram
• CASE 2: M:N, Both Sides OPTIONAL
Figure 4.51 Entity Relationships, M:N, Both Sides Optional
Developing an E-R
Diagram
• CASE 3: M:N, One Side OPTIONAL
Figure 4.52 Entity Relationships, M:N, One Side Optional
Developing an E-R
Diagram
• CASE 4: 1:M, Both Sides MANDATORY
Figure 4.53 Entity Relationships, 1:M, Both Sides Mandatory
Developing an E-R
Diagram
• CASE 5: 1:M, Both Sides OPTIONAL
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
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
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
Figure 4.57 Entity Relationships, 1:1, Both Sides Mandatory
Developing an E-R
Diagram
• CASE 9: 1:1, Both Sides OPTIONAL
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
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)
Figure 4.60 Entity Relationships, Weak Entity
Developing an E-R
Diagram
• CASE 12: Multivalued Attributes
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
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
– Documentation