LECTURE ON DATABASE Database Modal Systems

COPYRIGHT @ RAMEZ ELMASRI AND SHAMKANT B. NAVATHE
LECTURE 3
Data Modeling Using the
Entity-Relationship (ER) Model

OUTLINE
 Overview of Database Design Process
 Example Database Application (COMPANY)
 ER Model Concepts
 Entities and Attributes
 Entity Types, Value Sets, and Key Attributes
 Relationships and Relationship Types
 Weak Entity Types
 Roles and Attributes in Relationship Types
 ER Diagrams - Notation
 ER Diagram for COMPANY Schema
 Alternative Notations – UML class diagrams, others
 Relationships of Higher Degree

OVERVIEW OF DATABASE DESIGN PROCESS
 Two main activities:
 Database design
 Applications design
 Focus in this chapter on conceptual database design
 To design the conceptual schema for a database
application
 Applications design focuses on the programs and interfaces
that access the database
 Generally considered part of software engineering

OVERVIEW OF DATABASE DESIGN PROCESS

METHODOLOGIES FOR CONCEPTUAL DESIGN
 Entity Relationship (ER) Diagrams (This Chapter)
 Enhanced Entity Relationship (EER) Diagrams (Chapter 4)
 Use of Design Tools in industry for designing and
documenting large scale designs
 The UML (Unified Modeling Language) Class Diagrams are
popular in industry to document conceptual database
designs

EXAMPLE COMPANY DATABASE
 We need to create a database schema design based on the
following (simplified) requirements of the COMPANY
Database:
 The company is organized into DEPARTMENTs. Each
department has a name, number and an employee who
manages the department. We keep track of the start date
of the department manager. A department may have
several locations.
 Each department controls a number of PROJECTs. Each
project has a unique name, unique number and is located
at a single location.

EXAMPLE COMPANY DATABASE (CONTINUED)
 The database will store each EMPLOYEE’s social security
number, address, salary, sex, and birthdate.
 Each employee works for one department but may work
on several projects.
 The DB will keep track of the number of hours per week
that an employee currently works on each project.
 It is required to keep track of the direct supervisor of
each employee.
 Each employee may have a number of DEPENDENTs.
 For each dependent, the DB keeps a record of name, sex,
birthdate, and relationship to the employee.

ER MODEL CONCEPTS
 Entities and Attributes
 Entity is a basic concept for the ER model. Entities are specific things or objects in
the mini-world that are represented in the database.
 For example the EMPLOYEE John Smith, the Research DEPARTMENT, the ProductX
PROJECT
 Attributes are properties used to describe an entity.
 For example an EMPLOYEE entity may have the attributes Name, SSN, Address, Sex,
BirthDate
 A specific entity will have a value for each of its attributes.
 For example a specific employee entity may have Name='John Smith',
SSN='123456789', Address ='731, Fondren, Houston, TX', Sex='M', BirthDate='09-JAN-
55‘
 Each attribute has a value set (or data type) associated with it – e.g. integer,
string, date, enumerated type, …

TYPES OF ATTRIBUTES (1)
 Simple
 Each entity has a single atomic value for the attribute. For example, SSN or Sex.
 Composite
 The attribute may be composed of several components. For example:
 Address(Apt#, House#, Street, City, State, ZipCode, Country), or
 Name(FirstName, MiddleName, LastName).
 Composition may form a hierarchy where some components are themselves
composite.
 Multi-valued
 An entity may have multiple values for that attribute. For example, Color of a CAR
or PreviousDegrees of a STUDENT.
 Denoted as {Color} or {PreviousDegrees}.

TYPES OF ATTRIBUTES (2)
 In general, composite and multi-valued attributes may be nested
arbitrarily to any number of levels, although this is rare.
 For example, PreviousDegrees of a STUDENT is a composite multi-
valued attribute denoted by {PreviousDegrees (College, Year, Degree,
Field)}
 Multiple PreviousDegrees values can exist
 Each has four subcomponent attributes:
 College, Year, Degree, Field

EXAMPLE OF A COMPOSITE ATTRIBUTE

ENTITY TYPES AND KEY ATTRIBUTES (1)
 Entities with the same basic attributes are grouped or typed
into an entity type.
 For example, the entity type EMPLOYEE and PROJECT.
 An attribute of an entity type for which each entity must have
a unique value is called a key attribute of the entity type.
 For example, SSN of EMPLOYEE.

ENTITY TYPES AND KEY ATTRIBUTES (2)
 A key attribute may be composite.
 VehicleTagNumber is a key of the CAR entity type with components
(Number, State).
 An entity type may have more than one key.
 The CAR entity type may have two keys:
 VehicleIdentificationNumber (popularly called VIN)
 VehicleTagNumber (Number, State), aka license plate number.
 Each key is underlined (Note: this is different from the relational schema
where only one “primary key is underlined).

ENTITY SET
 Each entity type will have a collection of entities stored in the database
 Called the entity set or sometimes entity collection
 Previous slide shows three CAR entity instances in the entity set for
CAR
 Same name (CAR) used to refer to both the entity type and the entity
set
 However, entity type and entity set may be given different names
 Entity set is the current state of the entities of that type that are stored
in the database

VALUE SETS (DOMAINS) OF ATTRIBUTES
 Each simple attribute is associated with a value set
 E.g., Lastname has a value which is a character string of
upto 15 characters, say
 Date has a value consisting of MM-DD-YYYY where each
letter is an integer
 A value set specifies the set of values associated with an
attribute

DISPLAYING AN ENTITY TYPE
 In ER diagrams, an entity type is displayed in a rectangular
box
 Attributes are displayed in ovals
 Each attribute is connected to its entity type
 Components of a composite attribute are connected to
the oval representing the composite attribute
 Each key attribute is underlined
 Multivalued attributes displayed in double ovals
 See the full ER notation in advance on the next slide

NOTATION FOR ER DIAGRAMS

ENTITY TYPE CAR WITH TWO KEYS AND A
CORRESPONDING ENTITY SET

INITIAL CONCEPTUAL DESIGN OF ENTITY TYPES
FOR THE COMPANY DATABASE SCHEMA
 Based on the requirements, we can identify four initial entity types in the
COMPANY database:
 DEPARTMENT
 PROJECT
 EMPLOYEE
 DEPENDENT
 Their initial conceptual design is shown on the following slide
 The initial attributes shown are derived from the requirements
description

INITIAL DESIGN OF ENTITY TYPES:
EMPLOYEE, DEPARTMENT, PROJECT, DEPENDENT

REFINING THE INITIAL DESIGN BY INTRODUCING
RELATIONSHIPS
 The initial design is typically not complete
 Some aspects in the requirements will be represented as
relationships
 ER model has three main concepts:
 Entities (and their entity types and entity sets)
 Attributes (simple, composite, multivalued)
 Relationships (and their relationship types and
relationship sets)
 We introduce relationship concepts next

RELATIONSHIPS AND RELATIONSHIP TYPES (1)
 A relationship relates two or more distinct entities with a
specific meaning.
 For example, EMPLOYEE John Smith works on the ProductX
PROJECT, or EMPLOYEE Franklin Wong manages the Research
DEPARTMENT.
 Relationships of the same type are grouped or typed into a
relationship type.
 For example, the WORKS_ON relationship type in which
EMPLOYEEs and PROJECTs participate, or the MANAGES
relationship type in which EMPLOYEEs and DEPARTMENTs
participate.
 The degree of a relationship type is the number of
participating entity types.
 Both MANAGES and WORKS_ON are binary relationships.

RELATIONSHIP INSTANCES OF THE WORKS_FOR N:1
RELATIONSHIP BETWEEN EMPLOYEE AND DEPARTMENT

RELATIONSHIP INSTANCES OF THE M:N WORKS_ON
RELATIONSHIP BETWEEN EMPLOYEE AND PROJECT

RELATIONSHIP TYPE VS. RELATIONSHIP SET (1)
 Relationship Type:
 Is the schema description of a relationship
 Identifies the relationship name and the participating
entity types
 Also identifies certain relationship constraints
 Relationship Set:
 The current set of relationship instances represented in
the database
 The current state of a relationship type

RELATIONSHIP TYPE VS. RELATIONSHIP SET (2)
 Previous figures displayed the relationship sets
 Each instance in the set relates individual participating
entities – one from each participating entity type
 In ER diagrams, we represent the relationship type as follows:
 Diamond-shaped box is used to display a relationship type
 Connected to the participating entity types via straight lines
 Note that the relationship type is not shown with an arrow.
The name should be typically be readable from left to right
and top to bottom.

REFINING THE COMPANY DATABASE SCHEMA BY
INTRODUCING RELATIONSHIPS
 By examining the requirements, six relationship types are identified
 All are binary relationships( degree 2)
 Listed below with their participating entity types:
 WORKS_FOR (between EMPLOYEE, DEPARTMENT)
 MANAGES (also between EMPLOYEE, DEPARTMENT)
 CONTROLS (between DEPARTMENT, PROJECT)
 WORKS_ON (between EMPLOYEE, PROJECT)
 SUPERVISION (between EMPLOYEE (as subordinate), EMPLOYEE (as
supervisor))
 DEPENDENTS_OF (between EMPLOYEE, DEPENDENT)

ER DIAGRAM – RELATIONSHIP TYPES ARE:
WORKS_FOR, MANAGES, WORKS_ON, CONTROLS, SUPERVISION, DEPENDENTS_OF

DISCUSSION ON RELATIONSHIP TYPES
 In the refined design, some attributes from the initial entity
types are refined into relationships:
 Manager of DEPARTMENT -> MANAGES
 Works_on of EMPLOYEE -> WORKS_ON
 Department of EMPLOYEE -> WORKS_FOR
 etc
 In general, more than one relationship type can exist between
the same participating entity types
 MANAGES and WORKS_FOR are distinct relationship types between
EMPLOYEE and DEPARTMENT
 Different meanings and different relationship instances.

CONSTRAINTS ON RELATIONSHIPS
 Constraints on Relationship Types
 (Also known as ratio constraints)
 Cardinality Ratio (specifies maximum participation)
 One-to-one (1:1)
 One-to-many (1:N) or Many-to-one (N:1)
 Many-to-many (M:N)
 Existence Dependency Constraint (specifies minimum
participation) (also called participation constraint)
 zero (optional participation, not existence-dependent)
 one or more (mandatory participation, existence-dependent)

MANY-TO-ONE (N:1) RELATIONSHIP

MANY-TO-MANY (M:N) RELATIONSHIP

RECURSIVE RELATIONSHIP TYPE
 A relationship type between the same participating entity type in
distinct roles
 Also called a self-referencing relationship type.
 Example: the SUPERVISION relationship
 EMPLOYEE participates twice in two distinct roles:
 supervisor (or boss) role
 supervisee (or subordinate) role
 Each relationship instance relates two distinct EMPLOYEE entities:
 One employee in supervisor role
 One employee in supervisee role

DISPLAYING A RECURSIVE RELATIONSHIP
 In a recursive relationship type.
 Both participations are same entity type in different roles.
 For example, SUPERVISION relationships between
EMPLOYEE (in role of supervisor or boss) and (another)
EMPLOYEE (in role of subordinate or worker).
 In following figure, first role participation labeled with 1 and
second role participation labeled with 2.
 In ER diagram, need to display role names to distinguish
participations.

A RECURSIVE RELATIONSHIP SUPERVISION`

RECURSIVE RELATIONSHIP TYPE IS: SUPERVISION
(PARTICIPATION ROLE NAMES ARE SHOWN)

WEAK ENTITY TYPES
 An entity that does not have a key attribute and that is identification-
dependent on another entity type.
 A weak entity must participate in an identifying relationship type with an
owner or identifying entity type
 Entities are identified by the combination of:
 A partial key of the weak entity type
 The particular entity they are related to in the identifying relationship
type
 Example:
 A DEPENDENT entity is identified by the dependent’s first name, and
the specific EMPLOYEE with whom the dependent is related
 Name of DEPENDENT is the partial key
 DEPENDENT is a weak entity type
 EMPLOYEE is its identifying entity type via the identifying relationship
type DEPENDENT_OF

ATTRIBUTES OF RELATIONSHIP TYPES
 A relationship type can have attributes:
 For example, HoursPerWeek of WORKS_ON
 Its value for each relationship instance describes the
number of hours per week that an EMPLOYEE works on a
PROJECT.
 A value of HoursPerWeek depends on a particular
(employee, project) combination
 Most relationship attributes are used with M:N
relationships
 In 1:N relationships, they can be transferred to the
entity type on the N-side of the relationship

EXAMPLE ATTRIBUTE OF A RELATIONSHIP TYPE:
HOURS OF WORKS_ON

NOTATION FOR CONSTRAINTS ON RELATIONSHIPS
 Cardinality ratio (of a binary relationship): 1:1, 1:N, N:1, or
M:N
 Shown by placing appropriate numbers on the
relationship edges.
 Participation constraint (on each participating entity type):
total (called existence dependency) or partial.
 Total shown by double line, partial by single line.
 NOTE: These are easy to specify for Binary Relationship
Types.

ALTERNATIVE (MIN, MAX) NOTATION FOR
RELATIONSHIP STRUCTURAL CONSTRAINTS:
 Specified on each participation of an entity type E in a relationship type R
 Specifies that each entity e in E participates in at least min and at most max
relationship instances in R
 Default (no constraint): min=0, max=n (signifying no limit)
 Must have min  max, min0, max 1
 Derived from the knowledge of mini-world constraints
 Examples:
 A department has exactly one manager and an employee can manage at
most one department.
 Specify (0,1) for participation of EMPLOYEE in MANAGES
 Specify (1,1) for participation of DEPARTMENT in MANAGES
 An employee can work for exactly one department, but a department can
have any number of employees.
 Specify (1,1) for participation of EMPLOYEE in WORKS_FOR
 Specify (1,n) for participation of DEPARTMENT in WORKS_FOR

THE (MIN, MAX) NOTATION FOR RELATIONSHIP CONSTRAINTS
Read the min,max numbers next to the entity type and
looking away from the entity type

COMPANY ER SCHEMA DIAGRAM USING (MIN, MAX)
NOTATION

ALTERNATIVE DIAGRAMMATIC NOTATION
 ER diagrams is one popular example for displaying database
schemas
 Many other notations exist in the literature and in various
database design and modeling tools
 Appendix A illustrates some of the alternative notations that
have been used
 UML class diagrams is representative of another way of
displaying ER concepts that is used in several commercial
design tools

SUMMARY OF NOTATION FOR ER DIAGRAMS

UML CLASS DIAGRAMS
 Represent classes (similar to entity types) as large rounded
boxes with three sections:
 Top section includes entity type (class) name
 Second section includes attributes
 Third section includes class operations (operations are not in
basic ER model)
 Relationships (called associations) represented as lines
connecting the classes
 Other UML terminology also differs from ER terminology
 Used in database design and object-oriented software
design
 UML has many other types of diagrams for software design

UML CLASS DIAGRAM FOR COMPANY DATABASE
SCHEMA

OTHER ALTERNATIVE DIAGRAMMATIC NOTATIONS

RELATIONSHIPS OF HIGHER DEGREE
 Relationship types of degree 2 are called binary
 Relationship types of degree 3 are called ternary and of
degree n are called n-ary
 In general, an n-ary relationship is not equivalent to n binary
relationships
 Constraints are harder to specify for higher-degree
relationships (n > 2) than for binary relationships

DISCUSSION OF N-ARY RELATIONSHIPS (N > 2)
 In general, 3 binary relationships can represent different
information than a single ternary relationship (see Figure
3.17a and b on next slide)
 If needed, the binary and n-ary relationships can all be
included in the schema design (see Figure 3.17a and b,
where all relationships convey different meanings)
 In some cases, a ternary relationship can be represented as
a weak entity if the data model allows a weak entity type to
have multiple identifying relationships (and hence multiple
owner entity types) (see Figure 3.17c)

EXAMPLE OF A TERNARY RELATIONSHIP

ANOTHER EXAMPLE: A UNIVERSITY DATABASE
 To keep track of the enrollments in classes and student
grades, another database is to be designed.
 It keeps track of the COLLEGEs, DEPARTMENTs within each
college, the COURSEs offered by departments, and
SECTIONs of courses, INSTRUCTORs who teach the sections
etc.
 These entity types and the relationships among these entity
types are shown on the next slide in Figure 3.20.

UNIVERSITY DATABASE CONCEPTUAL SCHEMA

ANOTHER EXAMPLE

CHAPTER SUMMARY
 ER Model Concepts: Entities, attributes, relationships
 Constraints in the ER model
 Using ER in step-by-step mode conceptual schema design
for the COMPANY database
 ER Diagrams - Notation
 Alternative Notations – UML class diagrams, others
 Binary Relationship types and those of higher degree.

LECTURE ON DATABASE Database Modal Systems

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