Entity-Relationship Modelling

• Entity Sets• Relationship Sets• Design Issues • Mapping Constraints • Keys• E-R DiagramA data model is a collection of tools for

describing data, data relationships, data semantics, and consistency constraints.– An Entity-Relationship (E-R) Model is a data

model.

Entity Sets• The entity-relationship (E-R) data model perceives the real world as a

collection of entities and relationships.• ER model is good for describing a database in logical level.• An entity consists of a set of attributes.

– Examples: entity people have attributes person-id (e.g. HKID nos.), names, and addresses.

• An entity has a subset of attributes whose values uniquely identify the entity.– Example: people (identified by person-ids), loans (identified by loan-

ids)– Question: Any example of using two or more attributes to identify an

entity?• An entity set is a set of entities of the same type that share the same

properties.– Example: The set of all persons who are customers at a given bank

(can be defined as an entity set called customer)– Why do they belong to the same type?

• ‘coz each of them has an account number at the given bank– Another example: The set of all employees of a bank (entity set:

employee)

Entity Sets customer and loancustomer-id customer- customer- customer- loan- amount name street city number

entity set entity set

Attributes• An entity is represented by a set of attributes. These attributes are

commonly shared by all members in an entity set.

• Domain – the set of permitted values for each attribute. Any examples?– E.g. attribute loan-number might be the set of all strings of the form

“L-n” where n is a positive integer.• Attribute types

– Simple and composite attributes.• E.g. composite attribute name: first-name, middle-name, and

last-name– Single-valued and multi-valued attributes

• E.g. multivalued attribute: phone-numbers– Derived attributes

• Can be computed from other attributes• E.g. age, given date of birth

e.g. customer = (customer-id, customer-name, customer-street, customer-city)

loan = (loan-number, amount)

Composite Attributes

Relationship Sets• A relationship is an association among two or more entities

(it is a mathematical relation among n 2 entities)• A relationship set is a subset of

U = {(e1, e2, … en) | e1 E1, e2 E2, …, en En}

where

– E1, E2, …, En are entity sets.

– (e1, e2, …, en) is a relationship

– Question: What’s the cardinality of U?

|E1| |E2| … |En|

Example:Hayes depositor A-102

customer entity relationship set account entity– (Hayes, A-102) is a relationship in the relationship set depositor

Relationship Set borrower

Attributes of a Relationship• A relationship may also have attributes (called descriptive attributes).• E.g. the depositor relationship set between entity sets customer and

account may have the attribute access-date

Degree of a Relationship Set

• The number of entity sets that associate with in each relationship in a relationship set.

• A relationship that involves two entity sets is binary (or degree two).• Most relationship sets of database system are binary in practice• Relationship sets may involve more than two entity sets.

– Example: A bank employee may have different job at different branch• Ternary relationship set among employee, job and branch• Relationship set: works-on employee job branch• Jones (employee) is a manager (job) at Perryridge (branch).• Jones (employee) is an auditor (job) at Downtown (branch).• Smith (employee) is a teller (job) at Downtown (branch).

Mapping Cardinalities• Express the number of entities

to which another entity can be associated via a relationship set.

• For a binary relationship set the mapping cardinality must be one of the following types:– One to one– One to many– Many to one– Many to many

• Note: Some elements in A and B may not be mapped to any elements in the other set

• Any example? One to one One to many

• E.g. A customer may have no account in the bank but only has a loan record.

Mapping Cardinalities

Many to one Many to many

Mapping Cardinalities affect ER Design

Refer to the relationship set depositor (which has a descriptive attribute access-date) on slide 8.

Can make access-date an attribute of account, instead of a relationship attribute, if each account can have only one customer I.e., the relationship from customer to account is one to many

Question: Can we do something similar for a relationship of many to one?