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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)
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. ER Diagrams are most often used to design or debug relational databases in the fields of software engineering,
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 the relationship.
The entity relationship diagram is a graphical representation of a conceptual structure of a problem domain being modeled. The ERD assists the database designer in identifying the data and the rules that will be represented and used in a database.
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
Entity: real-world object or thing with an independent existence and which is distinguishable from other objects. Examples are a person, car, customer, product, gene, book etc. Attributes: an entity is represented by a set of attributes (its descriptive properties), e.g., name, age, salary, price etc.
• A person cannot be stored (an entity), but his/her name can be stored (a data type element). • Most current DBMS have some predefined set of data types which they support. • It is possible to use non-standard types in ER-schemas (complicates the later logical design). Stefan Brass: Datenbanken I Universit¨at Halle, 2004
