Database Systems Instructor Name: Lecture-9

Contents Degree of Relationship Cardinality of Binary Relationship 2

Degree of Relationship Number of Participating Entity Types in a Relationship Types of Degrees of Relationship Unary Binary Ternary 3

Unary Relationship It is the relationship between Entity Instance of the same Entity Type, also called Recursive Relationship Usual Situations When Unary Relationship exist are: One to One Relationship, when an entity instance is associated with exactly one entity instance, for example, married to in PERSON entity type One to Many Relationship, when an entity instance is associated with more than one entity instance 4

Unary Relationship-Contd. To Establish a Sequence, Cycle or Priority List between entity instances Employee Manages Employees Courses are pre-requisite to other courses Categories with lower level of categories 5

Unary Relationship-Example 6

Binary Relationship Relationship between two Entity Types It is the most common type of relationship encountered in ER Diagram 7

Binary Relationship- Example 8

Ternary Relationship It is a simultaneous relationship among instances of THREE entity types 9

Ternary Relationship- Example 10

Ternary Relationship Caution: A Ternary relationship is not three binary relationship as the relationship supply with attribute cannot be associated with any of the given Entity Type With associative Entity Type the Ternary Relationship becomes: 11

Ternary Relationship- Example 12

Ternary Relationship – How to Resolve? Convert all ternary relationship to associative entities 13

Notations for Relationship Weak Relationship also called defining relationship 14

Relationship Constraints Limits the possible combination of entities that participate in the corresponding relationship set For Example, if company rule dictates that an employ can work for only one Manager, this is a constraint on relationship One student can be enrolled in many courses One student majors in one department WHY, relationship constraints? Better Manipulation of Data Enforce Constraints If not represented in conceptual model, the domain knowledge may be lost 15

Types of Cardinality on Binary Relationship One to one One to many ( or many to one) Many to many 16

Notations for Cardinality on Binary Relationship – Crow’s Foot Notation 17

One-to-One Relationship A single Entity Instance from Entity Type is associated with a single Entity Instance of another Entity Type 18

One-to-One Relationship - Example 19

One-to-Many Relationship A single Entity Instance in one Entity Type is associated with a multiple Entity Instances of another Entity Type Also, called Parent – Child Relationship OR Master– Detail Relationship (Oracle) 20

One-to-Many Relationship - Example 21

Many-to-Many Relationship Each Entity Instance in one Entity Type is associated with a multiple Entity Instances of another Entity Type; and vice versa. 22

Many-to-Many Relationship - Example 23

Cardinality Constraint on Binary Relationship Cardinality Constraint specifies: Number of Instances of One Entity Type that must be associated with each Instances of another Entity Type OR Maximum Number of Relationship Instances that an Entity can participate in. ATTRIBUTE ON RELATIONSHIPS Attribute on one-to-one relationship can be moved to either side of Entity Type Attribute on one-to-many relationship can only be moved to “many” side of the relationship 24

Cardinality Constraints – Contd. Cardinality means “count” and expressed as number Minimum Cardinality: The minimum number of instances of one Entity that may participate in a relationship with each Instance of another Entity Maximum Cardinality: The maximum number of instances of one Entity that may participate in a relationship with each Instance of another Entity 25

Cardinality means “count” and expressed as number Minimum Cardinality Cardinality means “count” and expressed as number Minimum Cardinality: The minimum number of instances of one Entity that may participate in a relationship with each Instance of another Entity 26

Minimum Cardinality – Contd. Minimum is generally stated as either ‘Zero’ or ‘One’ Zero (optional participation): participation in relationship by the entity is option One (Mandatory participation): Participation in relationship by the entity is mandatory 27

Minimum Cardinality – Example 28

Cardinality with Optional and Mandatory Participation – Example 29

Participation Constraint It defines whether the existence of an entity depends on its being related to another entity via relationship type. This is also alternative way to define minimum cardinality constraint and its implications (optional, mandatory) 30

Participation Constraint Types Total Participation Partial Participation 31

Case I – Total Participation If company policy states that Every Employee Must work for a department, Then Employee Entity Only Exist if it participate in at least ONE work-for relationship It is total participation as Every Entity Instance in the EMPLOYEE entity type must be related to a department via work-for relationship Total Participation is also called Existence Dependency 32

Case II – Partial Participation We do not expect every employee to manage a department, so the participation of employee in manage relationship is Partial Some or part of employees from employee set are related to some departments; and not all employees. 33

Maximum Cardinality Maximum Number of One Entity that may be associated with each instance of another Entity Crow’s Foot 34

Crow’s Foot - Notations 35

Relationships and Cardinalities – Mandatory Cardinality - Example 36

Relationships and Cardinalities –Optional Cardinality - Example 37

Cardinality Constraints on Ternary Relationship 38

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Assignment – What I was Expecting major Department offers Is double major allowed? Can a student have more than 1 advisor? Is joint appointment of faculty possible? Can two profs share to teach the same course? Can a professor teach more than one course? Can a professor stay without affiliated with a department? faculty Courses teaches Professor advisor enrollment Students 40

Erroneous Relationship Receives or summarize the data, transfer, translate or calculate data 41