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Some relationships involve multiple entities from the same entity set •e.g., spouse (two persons), games (two teams) •e.g., recursive relationships, such as supervises (two employees)
1->1, can go to either entity 1->N, can go to (1) entity February 11, 2018 Entity-Relationship (ER) Diagrams 33 STUDENT DEPT MINOR_D FACULTY TUTORS CHAIR_F MAJOR_D Tutor Tutee 1 N M N M Office It is important to know whether or not a student has completed each of his/her minor(s). Done Done N 1 1
logical links between two or more entities. E.g. the pair (Johanssen,Stockholm), is an instance in the relationship Residence. 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...
What is an ER diagram? An Entity Relationship (ER) Diagram is a type of flowchart that illustrates how “entities” such as people, objects or concepts relate to each other within a system.
The entities (how is an employee represented?). The attributes (how is salary represented?). The relationships (how is the fact that an employee works in a certain department represented?). Users interact directly with the data model. The DBMS hides the way data are physically stored in the machine.
Entity relationship diagrams (ERD) are widely used in database design and systems analysis to represent systems or problem domains. The ERD was introduced by Chen (1976) in early 1976. Teorey, Yang, and Fry (1986) present an extended ER model for relational database design.
In terms of DBMS, an entity is a table or attribute of a table in database, so by showing relationship among tables and their attributes, ER diagram shows the complete logical structure of a database. Lets have a look at a simple ER diagram to understand this concept. Why use ER Diagrams?
