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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)
Sketching the key components is an efficient way to develop a working database. Entity = “thing” or object. Entity set = collection of similar entities. Similar to a class in object-oriented languages. Attribute = property of (the entities of) an entity set. Entity set = rectangle.
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.
A row in a table (or, a tuple in a relation) describes an entity. A column in a table (or, an element in a tuple) represents an attribute of an entity. A relationship between two entities is expressed as common values in one or more columns of their respective tables. The relational model provides an open-ended collection of scalar
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