1. CS4222 Principles of Database System
4/18/2018
CS4222 Principles of Database System
3. Entity-Relationship model 2
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Outline Weak entity sets Relationships of higher degree
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Outline
Alternative (min, max) notation for relationship constraints
Weak entity sets
Relationships of higher degree
Design choices for ER conceptual design
Data Modeling Tools
In class exercises
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3. Alternative (min, max) notation for relationship constraints
4/18/2018
Alternative (min, max) notation for relationship 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
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4. Alternative (min, max) notation for relationship 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
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5. Alternative (min, max) notation for relationship constraints
4/18/2018
Alternative (min, max) notation for relationship constraints
Read the min,max numbers next to the entity type and looking away from the entity type
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6. Weak Entity Sets What are the weak entity sets?
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Weak Entity Sets
What are the weak entity sets?
Entities that do not have sufficient attributes to form a key
They need to “borrow” the primary key from another entity set to form a key.
Partial key of a weak entity set
The set of attributes of a weak entity set that uniquely identify a weak entity for a given owner entity
The identifying Owner
The entity set that “lends” the primary key to a weak entity set
Weak 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 entity type
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7. Weak Entity Sets The following constraints must hold:
Owner entity set and weak entity set must participate in a one-to-many relationship set, which is called identifying relationship set of the weak entity set
Weak entity set must have total participation in this identifying relationship set.
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8. Weak Entity Sets Example: DEPENDENT is a weak entity type
A DEPENDENT entity is identified by the dependent’s name, and the specific EMPLOYEE with whom the dependent is related
Name of DEPENDENT is the partial key
EMPLOYEE is its identifying entity type via the identifying relationship type DEPENDENT_OF
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9. Weak Entity Sets Rectangle with double lines Broken lines
Weak identity sets
Identifying relationship set
Broken lines
Partial key of the weak identity set
name
ssn
name
sex
birthdate 1 N
Employees
Dependents_of
Dependents
Dependents
relationship
sex
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10. 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)
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11. 4/18/2018
ER DIAGRAM – Relationship Types are: WORKS_FOR, MANAGES, WORKS_ON, CONTROLS, SUPERVISION, DEPENDENTS_OF
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12. Discussion on Relationship Types
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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.
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13. COMPANY ER Schema Diagram using (min, max) notation
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COMPANY ER Schema Diagram using (min, max) notation
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14. Relationships of Higher Degree
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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
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15. Example: a ternary relationship
Supplier Supply part for a project
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16. 3. ER modeling CS4222_Su17

17. Discussion of n-ary relationships (n > 2)
In general, 3 binary relationships can represent different information than a single ternary relationship
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18. Discussion of n-ary relationships (n > 2)
Suppose that Can_Supply, includes an instance (s,p) whenever supplier s can supply part p (to any project)
Uses includes an instance (j,p) whenever a project j uses part p
Supplies includes an instance (s, j) whenever supplier s supplies some part to project j
The existence of the 3 relationship instances (s,p), (j,p), and (s,j) does not necessarily imply that an instance (s,j,p) exits in the ternary relationship Supply, because the meaning is different
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19. Discussion of n-ary relationships (n > 2)
If needed, the binary and n-ary relationships can all be included in the schema design
If only binary relationship is allowed, a ternary relationship such as Supply must be represented as a weak entity without partial key and with three identifying relationships
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20. Ternary relationship as a weak entity
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21. Design choices for ER conceptual design
The conceptual design is an iterative refinement process
The initial design is created and then iteratively refined until the most suitable design is reached
Refinements
A concept may be first modeled as an attribute and then refined into a relationship if the attribute is a reference to another entity type
An attribute that exists in several entity type may be promoted to an independent entity type
Eg. Suppose in the University database, several entity types suchas as Student, Instructor and Course
Each has an attribute Department in the initial design
An entity type department is created with a single attribute dept_name and is related to the three entity types via appropriate relationships
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22. Design choices for ER conceptual design
Refinements
An inverse refinement to the previous case may be applied
Eg. If an entity type exists in the initial design with a single attribute Dept_name and is related to only one other entity type, Student
In this case, Department may be reduced to an attribute of Student
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23. Summary of notation for ER diagrams
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24. 4/18/2018
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
NEGATIVES:
Most tools lack a proper distinct notation for relationships with relationship attributes
Mostly represent a relational design in a diagrammatic form rather than a conceptual ER-based design
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25. Some of the Currently Available Automated Database Design Tools
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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, space and security management
Oracle
Developer 2000/Designer 2000
Database modeling, application development
Popkin Software
System Architect 2001
Data modeling, object modeling, process modeling, structured analysis/design
Platinum (Computer Associates)
Enterprise Modeling Suite: Erwin, BPWin, Paradigm Plus
Data, process, and business component modeling
Persistence Inc.
Pwertier
Mapping from O-O to relational model
Rational (IBM)
Rational Rose
UML Modeling & application generation in C++/JAVA
Resolution Ltd.
Xcase
Conceptual modeling up to code maintenance
Sybase
Enterprise Application Suite
Data modeling, business logic modeling
Visio
Visio Enterprise
Data modeling, design/reengineering Visual Basic/C++
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26. 4/18/2018
Exercise 1
Design a DB representing cities, counties, and states in the US:
For states, record the name, population, and state capital (a city).
For counties, record the name, the population, and the located state.
For cities, record the name, the population, the located state and the located county.
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27. Design 1 Problem: County Population is repeated for each city. 1 1 1
Co. Popu.
Co. name
Popu. 1
cities
Located
states
name
Ci. Popu.
Ci. name 1 1
capital
Problem: County Population is repeated for each city.
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28. Design 2
Co. Popu.
Co. name
Popu.
name 1
counties
Located
states 1
Belongs-to 1 1
capitals
cities
The population of a county is derived from those of its cities.
Ci. Popu.
Ci. name
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29. Exercise 2 Consider the following ER diagram.
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30. Exercise 2
Assume that a course may or may not use a textbook, but that a text by definition is a book that is used in some course.
A course may not use more than five books.
Instructors teach from two to four courses.
Supply (min, max) constraints on this diagram.
State clearly any additional assumptions you make.
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