Entity-Relationship Model

Entity-Relationship Model

Entity/Relationship Modelling
Example In a University database we might have entities for Students, Modules and Lecturers. Students might have attributes such as their ID, Name, and Course, and could have relationships with Modules (enrolment) and Lecturers (tutor/tutee) Entities - Real-world object distinguishable from other objects Attributes - properties of an entity Relationships - links between entities Entity Relationship Modelling

Entity/Relationship Diagrams
Conceptual design Build a model independent of the choice of DBMS E/R Models are often represented as E/R diagrams that Give a conceptual view of the database Are independent of the choice of DBMS Faculty ID Name Course Teaches Student Studies Module Entity Relationship Modelling

E-R Diagrams Rectangles represent entity sets.
Diamonds represent relationship sets. Lines link attributes to entity sets and entity sets to relationship sets. Ellipses represent attributes Double ellipses represent multivalued attributes. Dashed ellipses denote derived attributes. Underline indicates primary key attributes

Other way of representation
Entity Relationship Modelling

Entity Relationship Modelling
An entity is represented by a set of attributes, that is descriptive properties possessed by all members of an entity set. Example: customer = (customer-id, customer-name, customer-street, customer-city) loan = (loan-number, amount) Entity Relationship Modelling

Entity Set: A collection of similar entities. E.g., all employees. Each entity set has a key. Each attribute has a domain. Domain – the set of permitted values for each attribute Eg: the names given for the attribute column ‘name’. Entity Relationship Modelling

Simple attribute – An attribute with single value, cannot be sub divided. Composite attribute – An attribute which can be sub divided. Null attribute – When an entity does not have a value for an attribute null value is used. Eg: An employee has no dependent, then dependent_name column will be null (value unknown) Entity Relationship Modelling

E-R Diagram With Composite, Multivalued, and Derived Attributes

Roles Entity sets of a relationship need not be distinct
The labels “manager” and “worker” are called roles; they specify how employee entities interact via the works-for relationship set. Roles are indicated in E-R diagrams by labeling the lines that connect diamonds to rectangles. Role labels are optional, and are used to clarify semantics of the relationship

Relationships Relationships are an association between two or more entities Each Student takes several Modules Each Module is taught by a Lecturer Each Employee works for a single Department Relationships have A name A set of entities that participate in them A degree - the number of entities that participate (most have degree 2) A cardinality ratio Entity Relationship Modelling

Cardinality Constraints

One-To-Many Relationship

Many-To-One Relationships
In a many-to-one relationship a loan is associated with several (including 0) customers via borrower, a customer is associated with at most one loan via borrower

Many-To-Many Relationship

Consider Works_In: An employee can work in many departments; a dept can have many employees. Entity Relationship Modelling

Making E/R Models To make an E/R model you need to identify Enitities Attributes Relationships Cardinality ratios General guidelines Since entities are things or objects they are often nouns in the description Attributes are facts or properties, and so are often nouns also Verbs often describe relationships between entities Entity Relationship Modelling

Example A university consists of a number of departments. Each department offers several courses. A number of modules make up each course. Students enrol in a particular course and take modules towards the completion of that course. Each module is taught by a lecturer from the appropriate department, and each lecturer tutors a group of students Entity Relationship Modelling

Example - Entities A university consists of a number of departments. Each department offers several courses. A number of modules make up each course. Students enrol in a particular course and take modules towards the completion of that course. Each module is taught by a lecturer from the appropriate department, and each lecturer tutors a group of students Entity Relationship Modelling

Example - Relationships
A university consists of a number of departments. Each department offers several courses. A number of modules make up each course. Students enrol in a particular course and take modules towards the completion of that course. Each module is taught by a lecturer from the appropriate department, and each lecturer tutors a group of students Entity Relationship Modelling

Example - E/R Diagram Entities: Department, Course, Module, Lecturer, Student Department Course Module Lecturer Student Entity Relationship Modelling

Example - E/R Diagram Each department offers several courses Offers Department Course Module Lecturer Student Entity Relationship Modelling

Example - E/R Diagram A number of modules make up each courses Offers Department Includes Course Module Lecturer Student Entity Relationship Modelling

Example - E/R Diagram Students enrol in a particular course Offers Department Includes Course Module Lecturer Enrols In Student Entity Relationship Modelling

Example - E/R Diagram Students … take modules Offers Department Includes Course Module Lecturer Takes Enrols In Student Entity Relationship Modelling

Example - E/R Diagram Each module is taught by a lecturer Offers Department Includes Teaches Course Module Lecturer Takes Enrols In Student Entity Relationship Modelling

Example - E/R Diagram a lecturer from the appropriate department Offers Employs Department Includes Teaches Course Module Lecturer Takes Enrols In Student Entity Relationship Modelling

Example - E/R Diagram each lecturer tutors a group of students Offers Employs Department Includes Teaches Course Module Lecturer Takes Enrols In Tutors Student Student Entity Relationship Modelling

Example - E/R Diagram Offers Employs Department Includes Teaches Course Module Lecturer Takes Enrols In Tutors Student Entity Relationship Modelling

Entity-Relationship Model

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Entity-Relationship Model

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