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Session 2 Welcome: To the fourth learning sequence “ Entity-Relationship Model “ E-R Model Recap : 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 Model1 E-R Diagrams
Purpose of E/R Model • The 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 Sets • Entity = “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 Diagrams • In 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.
name Adviser Student Example • Entity set Student has two attributes, name and Adviser. • Each Student entity has values for these two attributes, e.g. (Ahmad, Kazem)
Relationships • A relationship connects two or more entity sets. • It is represented by a diamond, with lines to each of the entity sets involved.
name addr name code Teachers Courses teaches - Teachers are teaching some courses. Number - Students take some courses. Advises Takes - Teachers advises some students. Students name addr Example
Relationship Set • The 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.
Example • For the relationship Teaches, we might have a relationship set like: Teachers Courses Ahmad C++ Samir Math Ahmad Java Amer Internet Samir Algorithms Jaber Math Ramez C++
Multiway Relationships • Sometimes, 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.
Example name addr name code Teachers Courses number Preferences Students name addr
A Typical Relationship Set Teacher Student Course Ahmad Issam C++ Ahmad Nour C++ Ahmad Nour Java Amer Issam Internet Samir Samar Math Samir Issam Math Samir Nader Algorithms
Many-Many Relationships • Think 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 Relationship A customer is associated with several (possibly 0) loans via borrower A loan is associated with several (possibly 0) customers via borrower
Many-One Relationships • Some 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
Example • Advises, 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 relationship 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
One-One Relationships • In 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 “Multiplicity” • Show a many-one relationship by an arrow entering the “one” side. • Show a one-one relationship by arrows entering both entity sets.
Example Likes Students Courses Favorite
Attributes on Relationships • Sometimes it is useful to attach an attribute to a relationship. • Think of this attribute as a property of tuples in the relationship set.
Example Takes Students Course Date Date is a function of both the student and the course, not of one alone.
Roles • Sometimes an entity set appears more than once in a relationship. • Label the edges between the relationship and the entity set with names called roles.
Relationship Set Husband Wife Bob Ann Joe Sue … … Married husband wife Teachers Example
E-R Diagram Summary: In this learning sequence, we discussed the principles of Entity- Relationship diagram.