We store each employee’s name (first, last, MI), Social Security number (SSN), street address, salary, sex (gender), and birth date. An employee is assigned to one department, but may work on several projects, which are not necessarily controlled by the same department.
Entity-Relationship (ER) model •Popular high-level conceptual data model ER diagrams •Diagrammatic notation associated with the ER model 2
E/R Diagrams In an entity-relationship diagram: Entity set = rectangle. Attribute = oval, with a line to the rectangle representing its entity set.
Entity-Relationship Model Contents Basic concepts: entities and entity types, attributes and keys, relationships and relationship types Entity-Relationship schema (aka ER diagram) Constraints on relationship types Design choices Enhanced Entity-Relationship model features Steps in designing an ER schema Translation of an ER schema to tables
E-R model is a static concept, involving the structure of data and not the operations on data. E-R model is used to design a database. The database is not implemented. Therefore, the instance of E-R diagram never exists in the sense that a relation’s instances exist in a relational model.
What Does An ER Diagram Really Mean? Course and Room are entities. Meets is a relationship. Its instances describe particular meetings. An entity can have relationships with itself... If the relationship is not symmetric... ...need to indicate the two roles that the entity plays in …
according to the ER approach can be represented by a diagrammatic technique called an Entity-Relationship diagram. Notation: - entity type rectangle - relationship type diamond - attribute ellipse m : 1 (single valued attribute)
Entity Relationship (ER) modelling • is a design tool • is a graphical representation of the database system • provides a high-level conceptual data model • supports the user's perception of the data • is DBMS and hardware independent • had many variants • is composed of entities, attributes, and relationships
What is an Entity Relationship Diagram (ER Diagram)? An ER diagram shows the relationship among entity sets. An entity set is a group of similar entities and these entities can have attributes.
In this paper, therefore, we present step-by-step guidelines, a set of decision rules proven to be useful in building ER diagrams, and a case study problem with a preferred answer as well as a set of incorrect diagrams for the problem.