Upload
jonathan-parker
View
215
Download
1
Embed Size (px)
Citation preview
Topic 3
Data Modeling Using the Entity-Relationship (ER) Model
Faculty of Information Science and TechnologyMahanakorn University of Technology
Topic 3-2Mahanakorn University of TechnologyGraduate School of IT
Topic Outline
Definitions of TerminologyExample 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 - NotationER Diagram for COMPANY SchemaAlternative Notations – UML class diagrams, others
Topic 3-3Mahanakorn University of TechnologyGraduate School of IT
Terminology
Database ApplicationDatabase Application: a particular a particular databasedatabase and the associated programsthe associated programs (application programsapplication programs) ) that implement the database queries and updates
Entity-Relationship (ER) model: a popular high-level conceptual data model
Universal Modeling Language (UML): an object-oriented modeling methodology
Topic 3-4Mahanakorn University of TechnologyGraduate School of IT
FIGURE 3.1A simplified diagram to
illustrate the main phases of
database design.
Topic 3-5Mahanakorn University of TechnologyGraduate School of IT
Example COMPANY Database
Requirements of the Company (oversimplified for illustrative purposes)– 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.
– Each department controls a number of PROJECTs. Each project has a name, number and is located at a single location.
Topic 3-6Mahanakorn University of TechnologyGraduate School of IT
Example COMPANY Database (Cont.)
–We store each EMPLOYEE’s social security number, address, salary, sex, and birthdate. Each employee works for one department but may work on several projects. We keep track of the number of hours per week that an employee currently works on each project. We also keep track of the direct supervisor of each employee.
–Each employee may have a number of DEPENDENTs. For each dependent, we keep track of their name, sex, birthdate, and relationship to employee.
Topic 3-7Mahanakorn University of TechnologyGraduate School of IT
FIGURE 3.2An ER schema diagram for the COMPANY database.
Topic 3-8Mahanakorn University of TechnologyGraduate School of IT
ER Model Concepts
Entities and Attributes– Entities are specific objects or things 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 a 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, subrange, enumerated type, …
Topic 3-9Mahanakorn University of TechnologyGraduate School of IT
FIGURE 3.3Two entities, employee e1 and company c1,
and their attributes.
Topic 3-10Mahanakorn University of TechnologyGraduate School of IT
Types of Attributes (1)
Simple (Atomic)– 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}.
Topic 3-11Mahanakorn University of TechnologyGraduate School of IT
Types of Attributes (2)Derived
– A derived attribute is an attribute that is derivable from other attributes or related entities. For example, the Age attribute is derivable from the Birthdate attribute, or the NumberOfEmployee of a department entity can be derived by counting the number of employee working for that department.
Null Values– Missing, Not Applicable, Unknown
Complex– 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)}.
Topic 3-12Mahanakorn University of TechnologyGraduate School of IT
FIGURE 3.4A hierarchy of composite attributes.
Topic 3-13Mahanakorn University of TechnologyGraduate School of IT
FIGURE 3.5A complex attribute: AddressPhone.
Topic 3-14Mahanakorn University of TechnologyGraduate School of IT
Entity Types and Key Attributes Entities with the same basic attributes are grouped or typed into an entity
type. For example, the EMPLOYEE entity type or the PROJECT entity type.
The collection of all entities of a particular entity type in the database at any point in time is called an entity set. The entity set is usually referred to using the same name as the entity type.
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.
A key attribute may be composite. For example, VehicleTagNumber is a key of the CAR entity type with components (Number, State).
An entity type may have more than one key. For example, the CAR entity type may have two keys:– VehicleIdentificationNumber (popularly called VIN) and– VehicleTagNumber (Number, State), also known as license_plate
number.
Topic 3-15Mahanakorn University of TechnologyGraduate School of IT
FIGURE 3.6Two entity types, EMPLOYEE and COMPANY, and some
member entities of each.
Topic 3-16Mahanakorn University of TechnologyGraduate School of IT
ENTITY SET corresponding to the ENTITY TYPE CAR
car1
((ABC 123, TEXAS), TK629, Ford Mustang, convertible, 1999, (red, black))car2
((ABC 123, NEW YORK), WP9872, Nissan 300ZX, 2-door, 2002, (blue))car3
((VSY 720, TEXAS), TD729, Buick LeSabre, 4-door, 2003, (white, blue))
.
.
.
CARRegistration(RegistrationNumber, State), VehicleID, Make, Model, Year, (Color)
FIGURE 3.7Registration(RegistrationNumber, State), VehicleID,
Make, Model, Year, {color}
Topic 3-17Mahanakorn University of TechnologyGraduate School of IT
Value Sets (Domains)
A value set (or domain of values) specifies the set of values that may be assigned to the attribute for each individual entity.
Value sets are typically specified using the basic data types available in most programming languages.
A: E → P(V)
V = P(V1) × P(V2) × . . . × P(Vn) if A is composite
Topic 3-18Mahanakorn University of TechnologyGraduate School of IT
FIGURE 3.8Preliminary design of entity types for the COMPANY database.
There are several There are several implicit relationshipsimplicit relationships among among the various entity type. In the ER model, these the various entity type. In the ER model, these
references should not be represented as references should not be represented as attributes but as attributes but as relationshipsrelationships..
Topic 3-19Mahanakorn University of TechnologyGraduate School of IT
ER DIAGRAM – Entity Types are:EMPLOYEE, DEPARTMENT, PROJECT, DEPENDENT
Topic 3-20Mahanakorn University of TechnologyGraduate School of IT
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.
Topic 3-21Mahanakorn University of TechnologyGraduate School of IT
Example relationship instances of the WORKS_FOR relationship between EMPLOYEE and DEPARTMENT
e1
e2
e3
e4
e5
e6
e7
EMPLOYEE
r1
r2
r3
r4
r5
r6
r7
WORKS_FOR
d1
d2
d3
DEPARTMENT
Topic 3-22Mahanakorn University of TechnologyGraduate School of IT
Example relationship instances of the WORKS_ON relationship between EMPLOYEE and PROJECT
e1
e2
e3
e4
e5
e6
e7
r1
r2
r3
r4
r5
r6
r7
p1
p2
p3
r8
r9
Topic 3-23Mahanakorn University of TechnologyGraduate School of IT
Relationships and Relationship Types (2)
More than one relationship type can exist with the same participating entity types. For example, MANAGES and WORKS_FOR are distinct relationships between EMPLOYEE and DEPARTMENT, but with different meanings and different relationship instances.
Mathematically,– A relationship type R among n entity types E1, E2, …, En defines a set
of associations – or a relationship set – among entities from these entity types.
R = {ri | ri associates n individual entities (e1, e2, …, en)}, that is,
R E1 E2 … En
Topic 3-24Mahanakorn University of TechnologyGraduate School of IT
Relationship Degree, Role Names and Recursive Relationship
Degree of a Relationship Type– The number of participating entity types
Unary, binary, ternary and n-ary
Role Names and Recursive Relationships– The role name signifies the role that a participating
entity from the entity type plays in each relationship instance, and helps to explain what the relationship means.
– The role names become essential in recursive relationships.
Topic 3-25Mahanakorn University of TechnologyGraduate School of IT
FIGURE 3.10 Some relationship instances in the SUPPLY ternary relationship set.
Topic 3-26Mahanakorn University of TechnologyGraduate School of IT
FIGURE 3.11 A recursive relationship SUPERVISION between EMPLOYEE in the supervisor role (1) and
EMPLOYEE in the subordinate role (2).
2
221
2
12
1
21
1
2
21
Topic 3-27Mahanakorn University of TechnologyGraduate School of IT
ER DIAGRAM – Relationship Types are:WORKS_FOR, MANAGES, WORKS_ON, CONTROLS,
SUPERVISION, DEPENDENTS_OF
Topic 3-28Mahanakorn University of TechnologyGraduate School of IT
Constraints on Relationships (1)
Cardinality Ratios for Binary Relationships– The cardinality ratio for a binary relationship
specifies the maximum number of relationship instances that an entity can participate in. One-to-one (1:1) One-to-many (1:N) or Many-to-one (N:1) Many-to-many
Topic 3-29Mahanakorn University of TechnologyGraduate School of IT
FIGURE 3.12A 1:1 relationship, MANAGES.
Topic 3-30Mahanakorn University of TechnologyGraduate School of IT
FIGURE 3.13An M:N relationship, WORKS_ON.
Topic 3-31Mahanakorn University of TechnologyGraduate School of IT
Constraints on Relationships (2)
Participation Constraints and Existence Dependencies– The participation constraint specifies whether the
existence of an entity depends on its being related to another entity via the relationship type. This constraint specifies the minimum number of relationship instances that each entity can participate in (sometimes called the minimum cardinality constraint)Total participation (or existence dependency or mandatory
participation) (e.g., EMPLOYEE in WORKS_FOR)Partial participation (or optional participation) (e.g.,
EMPLOYEE in MANAGES)
Topic 3-32Mahanakorn University of TechnologyGraduate School of IT
Attributes of Relationship types
A relationship type can have attributes.Attributes of a 1:1 relationship type can be
migrated to one of the participating entity types.Attributes of a 1:N relationship type can be
migrated only to the entity type on the N-side of the relationship.
Attributes of a M:N relationship type cannot be moved to any participating entity type.
Topic 3-33Mahanakorn University of TechnologyGraduate School of IT
Attribute of a Relationship Type is: Hours of WORKS_ON
Topic 3-34Mahanakorn University of TechnologyGraduate School of IT
Weak Entity TypesAn entity that does not have a key attributeA weak entity must participate in an identifying relationship type
with an owner or identifying entity typeEntities are identified by the combination of:
– A partial key (discriminator) of the weak entity type– The particular entity they are related to in the identifying entity
typeExample: Suppose that a DEPENDENT entity is identified by the dependent’s first
name and birhtdate, and the specific EMPLOYEE that the dependent is related to. DEPENDENT is a weak entity type with EMPLOYEE as its identifying entity type via the identifying relationship type DEPENDENT_OF
Topic 3-35Mahanakorn University of TechnologyGraduate School of IT
Weak Entity Type is: DEPENDENTIdentifying Relationship is: DEPENDENTS_OF
Topic 3-36Mahanakorn University of TechnologyGraduate School of IT
SUMMARY OF ER-DIAGRAM NOTATION FOR ER SCHEMAS
Meaning
ENTITY TYPE
WEAK ENTITY TYPE
RELATIONSHIP TYPE
IDENTIFYING RELATIONSHIP TYPE
ATTRIBUTE
KEY ATTRIBUTE
MULTIVALUED ATTRIBUTE
COMPOSITE ATTRIBUTE
DERIVED ATTRIBUTE
TOTAL PARTICIPATION OF E2 IN R
CARDINALITY RATIO 1:N FOR E1:E2 IN R
STRUCTURAL CONSTRAINT (min, max) ON PARTICIPATION OF E IN R
Symbol
E1 R E2
E1 R E2
R(min,max)
E
N
Topic 3-37Mahanakorn University of TechnologyGraduate School of IT
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 Must have minmax, min0, 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 (0,n) for participation of DEPARTMENT in WORKS_FOR
Topic 3-38Mahanakorn University of TechnologyGraduate School of IT
The (min,max) notation relationship constraints
(1,1)(0,1)
(1,N)(1,1)
Topic 3-39Mahanakorn University of TechnologyGraduate School of IT
COMPANY ER Schema Diagram using (min, max) notation
Topic 3-40Mahanakorn University of TechnologyGraduate School of IT
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
Higher-order relationships discussed further in Topic 4
Topic 3-41Mahanakorn University of TechnologyGraduate School of IT
FIGURE 4.11Ternary
relationship types. (a) The SUPPLY relationship. (b)
Three binary relationships not
equivalent to SUPPLY. (c)
SUPPLY represented as a weak entity type.
Topic 3-42Mahanakorn University of TechnologyGraduate School of IT
FIGURE 4.12 Another example of ternary versus binary relationship types.
Topic 3-43Mahanakorn University of TechnologyGraduate School of IT
FIGURE 4.13 A weak entity type INTERVIEW with a ternary identifying relationship type.
Topic 3-44Mahanakorn University of TechnologyGraduate School of IT
Proper Naming of Schema Constructs
Singular names for entity typesUppercase letters for entity type and
relationship type namesCapitalize attribute namesLowercase letters for role namesChoose binary relationship names to make
the ER diagram of the schema readable from left to right and from top to bottom
Topic 3-45Mahanakorn University of TechnologyGraduate School of IT
ER Model Construct UML Construct Example (next slide)
Entity type Class EMPLOYEE, PROJECT
Entity Object Each employee
Attribute Attribute Name, SSn in EMPLOYEE
- Operation age, chane_project
Domain Domain Sex:{M,F}
Relationship type Association WORKS_FOR
Relationship instance Link -
Relationship attribute Link attribute StartDate in MANAGES
(min, max) notation Multiplicity 4..* or 1..1 or 0..1 etc.
Recursive relationship Reflexive association SUPERVISION
Relationship type Aggregation LOCATION/PROJECT
- Unidirectional/bidirectional -
Weak entity Qualified association (qualified aggregation)
DEPENDENT
Partial key Discriminator Dependent Name
Topic 3-46Mahanakorn University of TechnologyGraduate School of IT
FIGURE 3.16 The COMPANY conceptual scheme in UML class diagram notation.
Topic 3-47Mahanakorn University of TechnologyGraduate School of IT
Data Modeling Tools
A number of popular tools that cover conceptual modeling and mapping into relational schema design. Examples: ERWin, S- Designer (Enterprise Application Suite), ER- Studio, etc.
POSITIVES: serves as documentation of application requirements, easy user interface - mostly graphics editor support
Topic 3-48Mahanakorn University of TechnologyGraduate School of IT
Some of the Currently Available Automated Database Design Tools
COMPANY TOOL FUNCTIONALITY
Embarcadero Technologies
ER Studio Database Modeling in ER and IDEF1X
DB Artisan Database administration and space and security management
Oracle Developer 2000 and Designer 2000
Database modeling, application development
Popkin Software System Architect 2001 Data modeling, object modeling, process modeling, structured analysis/design
Platinum Technology
Platinum Enterprice Modeling Suite: Erwin, BPWin, Paradigm Plus
Data, process, and business component modeling
Persistence Inc. Pwertier Mapping from O-O to relational model
Rational Rational Rose Modeling in UML and application generation in C++ and JAVA
Rogue Ware RW Metro Mapping from O-O to relational model
Resolution Ltd. Xcase Conceptual modeling up to code maintenance
Sybase Enterprise Application Suite Data modeling, business logic modeling
Visio Visio Enterprise Data modeling, design and reengineering Visual Basic and Visual C++
Topic 3-49Mahanakorn University of TechnologyGraduate School of IT
ER DIAGRAM FOR A BANK DATABASE
© The Benjamin/Cummings Publishing Company, Inc. 1994, Elmasri/Navathe, Fundamentals of Database Systems, Second Edition
Topic 3-50Mahanakorn University of TechnologyGraduate School of IT
PROBLEM with ER notation
THE ENTITY RELATIONSHIP MODEL IN ITS ORIGINAL FORM DID NOT SUPPORT THE SPECIALIZATION/ GENERALIZATION ABSTRACTIONS
Topic 3-51Mahanakorn University of TechnologyGraduate School of IT
Extended Entity-Relationship (EER) Model
Incorporates Set-subset relationshipsIncorporates Specialization/Generalization Hierarchies
NEXT TOPIC ILLUSTRATES HOW THE ER MODEL CAN BE EXTENDED WITH
- Set-subset relationships and Specialization/Generalization Hierarchies and how to display them in EER diagrams