Entity Relationship Model Chapter 6

Basic Elements of E-R Model Entity Object of the real world that stores data. Eg. Customer, State, Project, Supplier, etc. Attribute A description of an entity. Eg. Worker has name, worker number, department, address, etc. Relationship

Example of an Attribute

Type of Attribute Simple Attributes vs. Composite Attributes

Solitary Attribute vs. Multiple Value Attribute

Derived Attribute

Attribute Domain & Key Set of values for an attribute Character Numeric Date Attributes with one key/ two keys

Relationship Link between entities. Types of relationship: Unary Binary Ternary

Unary

Binary

Ternary

Cardinality One to one relationship (1:1) One to many relationship (1:M) Many to many relationship (M:N)

1:1 Relationship

1:M Relationship

M:N Relationship

Relationship Participation Compulsory Optional

Guidelines and Steps in ER Model No system environment insertion.

Wrap-up on ERD Determine entities, relationship. Determine attributes. Determine attributes related to relationship (if any). Choose the keys for the entities. Determine the domain for each attribute. Combine the diagrams. Check and refine.

Tutorial 1 Prepare an ERD for the following case study: A university database contains information about lecturers (identified by staff number) and courses (identified by course code). Lecturers teach courses; each of the following situations concerns the Teaches relationship set. For each situation, create an Entity Relationship (ER) diagram that describes it (assuming no further constraints hold). 1. Lecturers can teach the same course in several semesters, and each course offering must be recorded. 2. Every lecturer must teach some course. 3. Every lecturer teaches exactly one course (no more, no less). 4. Every lecturer teaches exactly one course (no more, no less), and every course must be taught by some lecturer. 5. Now suppose that certain courses can be taught by a team of lecturers jointly, but it is possible that no one lecturer in a team can teach the course. Model this situation, introducing additional entity sets and relationship sets if necessary.

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