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Entity-relationship model
An entity-relationship model (ERM) is an abstract conceptual representation of structured data. Entity-relationship modeling is a relational schema database modeling method, used in software engineering to produce a type of conceptual data model (or semantic data model) of a system, often a relational database, and its requirements in a top-down fashion. Diagrams created using this process are called entity-relationship diagrams, or ER diagrams or ERDs for short. Originally proposed in 1976 by Peter Chen, many variants of the process have subsequently been devised. The first stage of information system design uses these models during the requirements analysis to describe information needs or the type of information that is to be stored in a database. The data modeling technique can be used to describe any ontology (i.e. an overview and classifications of used terms and their relationships) for a certain universe of discourse (i.e. area of interest). In the case of the design of an information system that is based on a database, the conceptual data model is, at a later stage (usually called logical design), mapped to a logical data model, such as the relational model; this in turn is mapped to a physical model during physical design. Note that sometimes, both of these phases are referred to as "physical design". There are a number of conventions for entity-relationship diagrams (ERDs). The classical notation is described in the remainder of this article, and mainly relates to conceptual modeling. There are a range of notations more typically employed in logical and physical database design, including IDEF1x (ICAM DEFinition Language) and dimensional modeling.
Connection
Two related entities 
An entity with an attribute 
A relationship with an attribute 
A sample ER diagram An entity may be defined as a thing which is recognised as being capable of an independent existence and which can be uniquely identified. An entity is an abstraction from the complexities of some domain. When we speak of an entity we normally speak of some aspect of the real world which can be distinguished from other aspects of the real world (Beynon-Davies, 2004). An entity may be a physical object such as a house or a car, an event such as a house sale or a car service, or a concept such as a customer transaction or order. Although the term entity is the one most commonly used, following Chen we should really distinguish between an entity and an entity-type. An entity-type is a category. An entity, strictly speaking, is an instance of a given entity-type. There are usually many instances of an entity-type. Because the term entity-type is somewhat cumbersome, most people tend to use the term entity as a synonym for this term. Entities can be thought of as nouns. Examples: a computer, an employee, a song, a mathematical theorem. Entities are represented as rectangles. A relationship captures how two or more entities are related to one another. Relationships can be thought of as verbs, linking two or more nouns. Examples: an owns relationship between a company and a computer, a supervises relationship between an employee and a department, a performs relationship between an artist and a song, a proved relationship between a mathematician and a theorem. Relationships are represented as diamonds, connected by lines to each of the entities in the relationship. The model's linguistic aspect described above is utilized in the database query language ERROL. Entities and relationships can both have attributes. Examples: an employee entity might have a Social Security Number (SSN) attribute; the proved relationship may have a date attribute. Attributes are represented as ellipses connected to their owning entity sets by a line. Every entity (unless it is a weak entity) must have a minimal set of uniquely identifying attributes, which is called the entity's primary key. Entity-relationship diagrams don't show single entities or single instances of relations. Rather, they show entity sets and relationship sets. Example: a particular song is an entity. The collection of all songs in a database is an entity set. The eaten relationship between a child and her lunch is a single relationship. The set of all such child-lunch relationships in a database is a relationship set. Lines are drawn between entity sets and the relationship sets they are involved in. If all entities in an entity set must participate in the relationship set, a thick or double line is drawn. This is called a participation constraint. If each entity of the entity set can participate in at most one relationship in the relationship set, an arrow is drawn from the entity set to the relationship set. This is called a key constraint. To indicate that each entity in the entity set is involved in exactly one relationship, a thick arrow is drawn. Associative entity is used to solve the problem of two entities with a many-to-many relationship http://infocom.cqu.edu.au/Courses/spr2000/95169/Extra_Examples/ERD.htm. http://infocom.cqu.edu.au/Courses/spr2000/95169/Extra_Examples/ERD.htm. Unary Relationships - a unary relationship is a relationship between the rows of a single table. Alternative diagramming conventionsCrow's Foot
Two related entities shown using Crow's Foot notation Three symbols are used to represent cardinality:
These symbols are used in pairs to represent the four types of cardinality that an entity may have in a relationship.
These are notations of a side in the ER diagrams. You can see an example of the Crow's Foot notation in the diagram above. In the diagram, the following facts are detailed:
Crow's foot notation has the following benefits:
See also
Proprietary ER diagramming tools
Free software ER diagramming toolsTools that can interpret and generate ER models, SQL and do database analysis.
Free software Diagram toolsThese are tools that can't create ER diagrams but just draw the shapes without having any knowledge of what they mean or generating SQL.
ReferencesFurther readingExternal links
de:Entity-Relationship-Modell el:??????? ?????????-??????????? es:Modelo entidad-relación fr:Modčle entité-relation ko:??-?? ?? it:Modello E-R nl:Entity-relationshipmodel ja:??????? pl:Entity Relationship Diagram pt:Diagrama entidade relacionamento ru:ER-?????? ?????? sr:????? ???????-???? th:??????????????????????????? zh:ER?? Source: Wikipedia | The above article is available under the GNU FDL. | Edit this article
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