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DBMS UNIT1
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Entity-Relationship (E-R) Model
COURSE# CTITILE PROF SCHED
ROOMCOURSE
ENROLL
STUDENT
STUID SNAME MAJOR CREDITS
GRADE
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Class Hierarchies
Employees
ssn
name
lot
ISA
HourlyEmps
ContractEmps
Hourlywage
Hoursworked contractid
* Sometimes it’s natural to classify the entities in an entity set into subclass, then we will use Class Hierarchies.
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Aggregationname
ssn lot
Employees
monitors until
since
Sponsors DepartmentProjects
pid
Started-on
pbudget
dname
did budget
* A relationship set is an
association between
entity sets.
Sometimes we have to model a relationship between a collection of entities and relationships, then we need to use aggregation.
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Conceptual design using the ER model
• Should a concept be modeled as an entity or an attribute ?
• Should a concept be modeled as an entity or a relationship ?
• What are the relationships ? Should we use binary or ternary relationships ?
• Should we use aggregation ?
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UNARY RELATIONSHIP• Unary relationship : is an associate of 1 entity set.
* NoteNote : an entity set is a group of related entities.
Student
* Entity set =
* Relation =
Dr. Lee
* Attribute =
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BINARY RELATIONSHIP
• Binary relationship : is an associate of 2 entities sets .
Employees Works_In Department
* Entity set =
* Relation =
* Attribute =
ssn name lot did budgetdname
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TERNARY RELATIONSHIP
Works_InEmployees Department
Locations
Ternary relationship : is an associate of 3 entities sets .
* Entity set =
* Relation =
ssn name lot did dname budget
capadd
* Attribute =
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QUARTERY RELATIONSHIP
Employees Works_In
Locations
Department
Product
Quartery relationship : is an associate of 4 entities sets.
* Entity set =
* Relation =
* Attribute =
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Additional features of the ER model
• In the ER model allow us to draw important distinctions about the data.
• One of the features of the ER model is :
Key Constraints
Note Note : A key constraints is also known as Primary key.
A primary key is a candidate key selected to uniquely identify all other attribute values in any given row, can not contain null entries.
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An example of Key Constraints on Manage
Manages DepartmentEmployees
ssn
name
lot
Since dname
did budget
ssn = Social Security Number.
did = Department id.
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Weak Entities
A weak entity : is an entity of which is existence depends on other entities.
Policy DepartmentEmployees
ssn
name
lot
cost
Pname---------
age
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Entity versus Attribute
Works-In2 DepartmentEmployees
ssn
name
lot
fromdname
did budget
to
* Entity set =
* Relation =
* Attribute =
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Entity versus Relationship
Manages DepartmentEmployees
ssn
name
apptnum
did budget
dname
Mgr-Appts
lot
since
dbudget
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Binary versus Ternary Relationship
Covers DependentsEmployees
ssn
name
Pname---------
age
Policies
lot
policyid cost
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Using a Ternary Relationship instead of Aggregation
Sponsors DepartmentsProjects
pid
Started-on
did budgetpbudget
dname
Employees
name
ssn lot
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ONE TO MANY RELATIONSHIP
customer
customer-name
customer-citycustomer-id
borrower
customer-street
loan
loan-number
amount
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MANY TO ONE RELATIONSHIP
customer
customer-name
customer-citycustomer-id
borrower
customer-street
loan
loan-number
amount
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ONE TO ONE RELATIONSHIP
customer
customer-name
customer-citycustomer-id
borrower
customer-street
loan
loan-number
amount
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E-R Diagram with Composite, Multivalued, and Derived Attributes
Customer
date-of-birthphone-number
name
customer-id
first-name
age
last-name
middle-name
zip-code
state
city
address
street
Street-number apartment-number
street-name
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WEAK ENTITY SETS
An entity set may not have sufficient attributes to form a primary key. Such an entity set is termed a weak entity set. An entity set that has a primary key is termed a strong entity set.
For a weak entity set to be meaningful, it must be associated with another entity set, called the identifying or owner entity set. The relationship associating the weak entity set with the identifying entity set is called the identifying relationship.
The identifying relationship is many to one from the weak entity set to the identifying entity set, and the participation of the weak entity set in the relationship is total.
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DISCRIMINATOR
The discriminator of a weak entity set is a set of attributes that allows this distinction to be made. For example, the discriminator of a weak entity set payment is the attribute payment-number, since, for each loan a payment number uniquely identifies one single payment for that loan. The discriminator of a weak entity set is also called the partial key of the entity set.
Note: although each payment entity is distinct, payments for different loans may share the same payment-number. Thus, payment entity set does not have a primary key; it is a weak entity set.
The primary key of a weak entity set is formed by the primary key of the identifying entity set, plus the weak entity set’s discriminator.
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E-R DIAGRAM WITH A WEAK ENTITY SET
In a E-R diagrams, a doubly outlined box indicates a weak entity set, and a doubly outlined diamond indicates the corresponding identifying relationship. We underline the discriminator of a weak entity set with a dashed line.
loan
E-R diagram with a weak entity set
loan-payment payment
payment-date
payment-amountpayment-number
amountloan-number
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Students
Entity/Relationship ModelDiagrams to represent designs.• Entity like object, = “thing.”• Entity set like class = set of “similar”
entities/objects.• Attribute = property of entities in an entity set,
similar to fields of a struct.• In diagrams, entity set rectangle;
attribute oval.ID name phone
height
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Relationships
• Connect two or more entity sets.
• Represented by diamonds.
Students CoursesTaking
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Relationship Set
Think of the “value” of a relationship set as a table.
• One column for each of the connected entity sets.
• One row for each list of entities, one from each set, that are connected by the relationship.
Students Courses
Sally CS180
Sally CS111
Joe CS180
… …
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Multiway RelationshipsUsually binary relationships (connecting two E.S.) suffice.• However, there are some cases where three or more E.S.
must be connected by one relationship.• Example: relationship among students, courses, TA's
(and graders).
Possibly, this E/R diagram is OK:
Students CoursesTaking
Assisting
TA/Graders
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• Works in CS180, because each TA (or grader) is a TA of all students. Connection student-TA is only via the course.
• But what if students were divided into sections, each headed by a TA? Then, a student in CS180 would be related to
only one of the TA's for CS180. Which one?
• Need a 3-way relationship to tell.
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Students Courses TAsAnn CS180 JanSue CS180 PatBob CS180 Jan… … …
Students
Courses
TAs
Enrolls
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Beers-Bars-Drinkers Example
• Our running example for the course.name addr license
name manf name addr
Beers Drinkers
BarsServes Frequents
Likes
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Multiplicity of Relationships
Representation of Many-One
• E/R: arrow pointing to “one.” Rounded arrow = “exactly one.”
Many-many Many-one One-one
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Example:Drinkers Have Favorite Beers
name addr license
name manf name addr
Beers Drinkers
BarsServes Frequents
Likes
Favorite
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One-One Relationships
Put arrows in both directions.
Design Issue:Is the rounded arrow justified?Design Issue:Here, manufacturer is an E.S.In earlier diagrams it is an attribute.Which is right?
Manfs BeersBest-seller
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Attributes on Relationships
• Shorthand for 3-way relationship:
Bars BeersSells
price
Bars BeersSells
price
Prices
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• A true 3-way relationship. Price depends jointly on beer and bar.
• Notice arrow convention for multiway relationships: “all other E.S. determine one of these.” Not sufficiently general to express any
possibility. However, if price, say, depended only on the
beer, then we could use two 2-way relationships: price-beer and beer-bar.
Or better: just make price an attribute of beer.
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Converting Multiway to 2-Way• Baroque in E/R, but necessary in certain “object-oriented” models.
• Create a new connecting E.S. to represent rows of a relationship set. E.g., (Joe's Bar, Bud, $2.50) for the Sells relationship.
• Many-one relationships from the connecting E.S. to the others.
Bars Beers
The-Bar
Price
The-Beer
The-Price
BBP
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Roles
Sometimes an E.S. participates more than once in a relationship.
• Label edges with roles to distinguish.
Husband Wifed1 d2
d3 d4
… …Drinkers
Married
husband wife
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• Notice Buddies is symmetric, Married not. No way to say “symmetric” in E/R.
Design Question
Should we replace husband and wife by one relationship spouse?
Buddy1 Buddy2d1 d2
d1 d3
d2 d1
d2 d4
… …Drinkers
Buddies
1 2
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More Design Issues
1. Subclasses.
2. Keys.
3. Weak entity sets. (Next class.)
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Subclasses
Subclass = special case = fewer entities = more properties.
• Example: Ales are a kind of beer. In addition to the properties (= attributes and relationships) of beers, there is a “color” attribute for ales.
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E/R Subclasses• Assume subclasses form a tree (no multiple
inheritance).• isa triangles indicate the subclass relation.
name manfBeers
Alescolor
isa
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Different Subclass Viewpoints1. E/R viewpoint: An entity has a component in each entity set
to which it logically belongs. Its properties are the union of the properties of these E.S.
2. Contrasts with object-oriented viewpoint: An object (entity) belongs to exactly one class. It inherits properties
of its superclasses.name manfBeers
Alescolor
isa Pete’s Ale
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Multiple InheritanceTheoretically, an E.S. could be a subclass of
several other entity sets.name manf
Beers
name manf
Wines
GrapeBeers
isaisa
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Problems
How should conflicts be resolved?• Example: manf means vintner for wines,
bottler for beers. What does manf mean for “grape beers”?
• Need ad-hoc notation to resolve meanings.• In practice, we shall assume a tree of entity
sets connected by isa, with all “isas” pointing from child to parent.
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Keys
A key is a set of attributes whose values can belong to at most one entity.
• In E/R model, every E.S. must have a key. It could have more than one key, but one set of
attributes is the “designated” key.
• In E/R diagrams, you should underline all attributes of the designated key.
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Example• Suppose name is key for Beers.
• Beer name is also key for ales. In general, key at root is key for all.
name manfBeers
Alescolor
isa
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Example: A Multiattribute Key
• Possibly, the combination of hours + room also forms a key, but we have not designated it as such.
dept roomCourses
number hours