*Session 2 Welcome: To the fourth learning sequence Entity-Relationship Model E-R ModelRecap : In the previous learning sequences, we discussed the basic definitions of Data base subject.Present learning: We shall explore the following definitions: - Purpose of E-R MODEL. - E-R Diagram. - Entity & Attributes. - Relations.

*Entity-Relationship Model1E-R Diagrams

*Purpose of E/R ModelThe E/R model allows us to sketch the design of a database informally.Designs are pictures called entity-relationship diagrams.Fairly mechanical ways to convert E/R diagrams to real implementations like relational databases exist.

*Entity SetsEntity = thing or object.Entity set = collection of similar entities.Similar to a class in object-oriented languages.Attribute = property of an entity set.Generally, all entities in a set have the same properties.Attributes are simple values, e.g. integers or character strings.

*E/R DiagramsIn an entity-relationship diagram, each entity set is represented by a rectangle.Each attribute of an entity set is represented by an oval, with a line to the rectangle representing its entity set.

*Example

Entity set Student has two attributes, name and Adviser.Each Student entity has values for these two attributes, e.g. (Ahmad, Kazem)

*RelationshipsA relationship connects two or more entity sets.It is represented by a diamond, with lines to each of the entity sets involved.

*Example

*Relationship SetThe current value of an entity set is the set of entities that belong to it.Example: the set of all students in our database.The value of a relationship is a set of lists of currently related entities, one from each of the related entity sets.

*ExampleFor the relationship Teaches, we might have a relationship set like:Teachers CoursesAhmad C++Samir MathAhmad JavaAmer InternetSamir AlgorithmsJaber MathRamez C++

*Multiway RelationshipsSometimes, we need a relationship that connects more than two entity sets.Suppose that students will only take certain courses taught by certain teachers.Our three binary relationships teaches, advises, and takes do not allow us to make this distinction.But a 3-way relationship would.

*ExampleTeachersCoursesStudentsnamenameaddrcodenameaddrnumberPreferences

*A Typical Relationship SetTeacherStudentCourseAhmadIssam C++Ahmad NourC++Ahmad NourJavaAmer IssamInternetSamir Samar Math Samir Issam MathSamir NaderAlgorithms

*Many-Many RelationshipsThink of a relationship between two entity sets, such as Teaches between Teachers and Courses.In a many-many relationship, an entity of either set can be connected to many entities of the other set.E.g., a teacher teaches many courses; a course is taught by many teachers.

*In Pictures:many-many

** Example of Many-Many RelationshipA customer is associated with several (possibly 0) loans via borrowerA loan is associated with several (possibly 0) customers via borrower

*Many-One RelationshipsSome binary relationships are many -one from one entity set to another.Each entity of the first set is connected to at most one entity of the second set.But an entity of the second set can be connected to zero, one, or many entities of the first set.

*In Pictures: many-one

*ExampleAdvises, from Teachers to Students is One-Many.A student has at most one adviser.But a teacher can be the adviser of any number of students, including zero.

** Example of Many-One relationshipIn 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

*One-One RelationshipsIn a one-one relationship, each entity of either entity set is related to at most one entity of the other set.Example: Relationship Is-in between entity sets Teachers and Offices.A teacher cannot be in more than one office, and no office can have more than one teacher (assume this).

*In Pictures: one-one

** Example of One-One relationship-A customer is associated with at most one loan via the relationship borrower A loan is associated with at most one customer via borrower

*Representing MultiplicityShow a many-one relationship by an arrow entering the one side.Show a one-one relationship by arrows entering both entity sets.

*ExampleStudentsCoursesLikesFavorite

*Attributes on RelationshipsSometimes it is useful to attach an attribute to a relationship.Think of this attribute as a property of tuples in the relationship set.

*ExampleStudentsCourseTakesDateDate is a function of both the student and the course, not of one alone.

*RolesSometimes an entity set appears more than once in a relationship.Label the edges between the relationship and the entity set with names called roles.

*Example

*E-R Diagram Summary: In this learning sequence, we discussed the principles of Entity- Relationship diagram.

*END