1. Entity-Relationship Model
Chapter 2
B.Ramamurthy
11/29/2018
B.Ramamurthy

2. Introduction
E-R model facilitates database design by allowing the specification of an “enterprise schema” which represents the overall logical structure of a database.
The E-R model is extremely useful in mapping the meanings and interactions of real-world enterprises onto a conceptual schema.
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B.Ramamurthy

3. Topics for Discussion Entity Sets Relationship Sets Design Issues
Mapping Constraints
Keys
E-R Diagram
Extended E-R Features
Design of an E-R Database Schema
Reduction of an E-R Scheme to Tables
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B.Ramamurthy

4. Entity A database can be modeled as : a collection of entities
relationships among entities.
An entity is an object that exists and is distinguishable from other objects.
Example: specific person, company, event, plant
More Specific example: Consider Federal Government as a large enterprise.
Entities : President, IRS, amendment5, senate
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B.Ramamurthy

5. Entity Sets
An entity set is a set of entities of the same type that share the same properties:
Examples: set of all persons, companies, trees, holidays
Considering Federal government :
senator, office, cherry-blossom, fed-holiday
11/29/2018
B.Ramamurthy

6. Attributes An entity is represented by a set of attributes.
Attributes are properties possessed by all members of an entity set.
Example:
Customer = (name, SSN, street, city)
account = (number, balance)
Consider federal government :
senator = (name, state, party)
amendment = (number, definition)
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B.Ramamurthy

7. Attributes (contd.)
Domain : the set of permitted values for each attribute
Example: domain of attribute “state” comprises the 50 states
Cardinality of a domain : 50 is the cardinality of state attribute
Attribute types:
simple and composite
single valued and multi-valued
null
derived
11/29/2018
B.Ramamurthy

8. Attribute Types : Examples
Single/composite : zip code is a single integer whereas street is composite attribute
Single/multi-valued: loan number is unique single values whereas dependent could be many.
Null : no values, starting point, indication of empty
Derived : Number of accounts held by a person can be derived (counted) and need not be physically represented wasting storage.
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B.Ramamurthy

9. Relationship A relationship is an association among entities. A-102
Account entity
Hayes
Customer entity
Depositor
Relationship
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B.Ramamurthy

10. Relationship Sets
A relationship set is a mathematical relation among n >= 2 entities, each taken from entity sets
{(e1,e2, ….en) | e1 Є E1, e2 Є E2, …en Є En}
where (e1,e2,..en) is a relationship,
E1, E2, .. En are entity sets
(Hayes, A-102) Є Depositor
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B.Ramamurthy

11. Attribute of a Relationship Set
entity
attribute
relationship
account
number
balance
customer
name
SSN
street
city
depositor
access-date
11/29/2018
B.Ramamurthy

12. Degree of Relationship Set
Refers to the number of entity sets that participate in a relationship set.
Relationship sets that involve two entity sets are binary. Generally most relationship sets in a database are binary.
Relationship sets may involve more than two entity sets: the entity customer, loan and branch may be linked by ternary relationship set.
N-nary relationship are also possible.
11/29/2018
B.Ramamurthy

13. Roles Entity set of a relationship need not be distinct. For example
phone
name
city
SSN
manager
employee
works-for
worker
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B.Ramamurthy

14. Roles (contd.) The labels “manager” and “worker” are called “roles”
They specify how employee entities interact via the “works-for” relationship set
Roles are indicated in ER diagrams by labeling the lines that connect diamonds to rectangles.
Roles are optional.
They clarify semantics of a relationship.
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B.Ramamurthy

15. Design Issue 1 Entity sets vs. Attributes
depends on enterprise being modeled and the semantics of the data
Example 1: An auto is a dealership could be attribute whereas in a manufacturing plant it is an entity set.
Example 2: The type of a number to an application program is an attribute but in a mathematical system it is en entity-set.
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B.Ramamurthy

16. Design Issue 2 Entity sets vs Relationship sets
Relationship set is to describe an action that occurs among entities.
Example 1: Author for a book company could be an entity-set whereas it is relationship set between entity-sets Person and Books
One of the ways to distinguish between entity-sets and relationships is to use connectives such “for” , “of” :
Example : “Author of” instead “Author” explicitly puts that as a relationship set than an entity set.
Example 2 : “Works for” instead of “worker”
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B.Ramamurthy

17. Design Issue 3 Binary vs n-ary relationship sets
Example: Lawyer, Business, Bank entity sets related by a relationship set Business-Transaction
Even though it is possible to replace a n-ary relationship set by a number of binary relationship sets, n-ary is more expressive.
Often n-ary to binary is done for ease of analysis.
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B.Ramamurthy

18. Exercises
Hwk1 : 2.3, 2.4, 2.5 (for 2.3 and 2.4 only) , 2.6 Due : 9/8/2000
Ex 2.2 page 59
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B.Ramamurthy

19. Mapping Cardinalities
The number of entities to which another entity can be associated via a relationship set.
One of the following types:
one to one (Ex: wife entity set, marriage relationship set, husband entity set)
one to many (Ex: teacher entity set, course relationship set, student entity set)
many to one (Ex: client entity set, law suit relationship set, company entity set)
many to many (Ex: people entity set, travel relationship entity, cities entity set)
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B.Ramamurthy

20. Existence Dependencies
If the existence of entity x depends on entity y, x is said to be dependent on y. (Ex: payment and loan)
Y is a dominant entity
X is a subordinate entity
In the example, if loan is deleted the payment should also go.
11/29/2018
B.Ramamurthy

21. Total and Partial Participation
If every entity in a entity set has at least one relation in a relationship set then it is known as “total” participation.
Ex: Employee entity set , salary-paid relationship set, salary-code entity set
If only some of the entities have relations in a relationship set then it is known as “partial” participation.
Ex: Employee entity set, on-leave relationship set, dates entity set
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B.Ramamurthy

22. Keys
A super key of an entity set is a set of one or more attributes whose values uniquely determine each entity.
A candidate key of an entity set is a minimal super key. Ex: SSN, account number
One of the candidate keys is selected to be the primary key.
Combination of primary keys of participating entity sets forms candidate key of a relationship set
Ex: (SSN, acctNo) is the primary key of depositor relationship set.
11/29/2018
B.Ramamurthy

23. E-R Diagram Components
Rectangles represent entity sets
Ellipses represent attributes
Diamonds represent relationship sets
Lines link attributes to entity sets, entity sets to relationship sets (also represent roles)
Double ellipses multi-valued attributes
Dashed ellipses denote derived value attributes
Primary key attributes are underlined.
11/29/2018
B.Ramamurthy

24. Weak Entity Sets
An entity set that does not have a primary key is referred to as a weak entity set.
The existence of a weak entity set is dependent on the existence of a strong entity set.
Discriminator or partial key of a weak entity set is the set of all attributes that distinguishes among all the entities of a weak entity set.
Primary key of strong entity set + discriminator is the primary key of a weak entity set.
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B.Ramamurthy

25. Weak Entity Set (contd.)
Use double rectangle to represent weak entity set.
Underline the discriminator of a weak entity set with dashed line.
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B.Ramamurthy

26. Weak Entity Set : Example
date
amount
number
amount
number
loan
loan-payment
payment
Weak entity set is non-existent is the corresponding strong entity set is removed.
11/29/2018
B.Ramamurthy

27. E-R Model to Table
Strong entity sets to tables : Reduces to a table with same attributes
Weak entity sets to tables : table that includes a column for primary key of identifying strong entity set.
Relationship sets to tables : has columns corresponding to primary keys of the entity sets and the attributes of the relationship set.
No need for table corresponding to weak relationship set.
An algorithm for conversion from ER to tables is discussed in the recitation.
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B.Ramamurthy

28. Multi-values Attributes
Multi-valued attributes result in a table.
But often a combination of tables can be reduced into fewer tables.
Ex: Account-balance.
Aggregation and Generalization relationships are also represented by a set of tables.
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B.Ramamurthy

29. E-R Diagram for the Banking Enterprise
Lets go through a complete example in Fig.2.18 and examine how to convert it to relational tables.
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B.Ramamurthy

30. Summary
E-R Model provides a means for analyzing and representing the database design for an enterprise.
E-R diagram can be systematically converted into a tables defining the logical structure of the database.
11/29/2018
B.Ramamurthy