UNARY RELATIONSHIP

1
UNARY RELATIONSHIP
Manages
Is Married to
EMPLOYEE
PERSON
One-to-One
One-to-Many
2
BINARY RELATIONSHIP
Is Assigned
EMPLOYEE
PARKING PLACE
One-to-one
Contains
PRODUCT LINE
PRODUCT
One-to-many
Registers for
STUDENT
COURSE
Many-to-many
3
TERNARY RELATIONSHIP
PART
Ships
VENDOR
WAREHOUSE
QUANTITY
4
GERUNDS

• A gerund (sometimes called a composite entity) is
  a many-to-many relationship that the data modeler
  chooses to model as an entity type with several
  associated one-to-many relationships.
• Example

5
GERUNDS
VENDOR
PART
SHIPMENT NO.
QUANTITY
SHIPMENT
WAREHOUSE
6
Cardinalities in Relationships

• The cardinality of a relationship is the number
  of instances of entity B that can (or must) be
  associated with each instance of entity A.
• Example

Is Stocked as
MOVIE COPY
MOVIE
7
Minimum and Maximum Cardinalities

• The minimum cardinality of a relationship is the
  minimum number of instances of entity B that may
  be associated with each instance of entity A.
• The maximum cardinality of a relationship is the
  maximum number of instances.
• Example

8
Has
PATIENT
PATIENT HISTORY
a) Mandatory cardinalities
Is Assigned to
EMPLOYEE
PROJECT
b) One optional, one mandatory cardinality
Is Married to
PERSON
c) Optional cardinalities
9
Existence Dependency and Weak Entity

• Existence dependency means that an instance of
  one entity cannot exist without the existence of
  an instance of some other (related) entity.
• A weak entity is an entity type that has an
  existence dependency.

10
Identifying Relationship

• An identifying relationship is one in which the
  primary key of the parent entity is used as part
  of the primary key of the dependent entity.
• Two benefits of an Identifying Relationship
• Data Integrity. Existence dependencies are
  enforced since the primary is shared.
• Ease of access of the dependent entity.
• Example

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MOVIE NO.
MOVIE NAME
COPY NO.
MOVIE NO.
Is Stocked as
MOVIE
MOVIE COPY
Identifying Relationships
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Summary of Symbols Used in E-R Notation

• Basic Symbols

ATTRIBUTE
ENTITY
PRIMARY KEY
RELATIONSHIP
MULTIVALUED ATTRIBUTE
GERUND
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RELATIONSHIP DEGREE
Unary
TERNARY
Is Assigned
EMPLOYEE
PARKING PLACE
Binary
14
Relationship Cardinality
Mandatory 1 cardinality
Many (M) cardinality (1,2, many)
Optional 0 or 1 cardinality
Optional zero-many cardinality (0,1,2,,many)
15
Class-subclass relationship
ISA
Exclusive relationship
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Alternative E-R Notation
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Alternative E-R Notation (contd.)
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Alternative E-R Notation (contd.)