Entity-relationship model Totally Explained

Everything about Entity Relationship Diagram totally explained

An entity-relationship model 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 for short. Originally proposed in 1976 by Dr. Pin-Shan (Peter) Chen (陳品山), 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's to be stored in a database. The data modeling technique can be used to describe any ontology (for example an overview and classifications of used terms and their relationships) for a certain universe of discourse (for example area of interest). In the case of the design of an information system that's 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

entity represents a discrete object. 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's 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're 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 (External Link

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Unary Relationships - a unary relationship is a relationship between the rows of a single table.

Alternative diagramming conventions

Crow's Feet

The "Crow's Foot" notation represents relationships with connecting lines between entities, and pairs of symbols at the ends of those lines to represent the cardinality of the relationship. Crow's Feet are used in Barker's Notation and in methodologies such as SSADM and Information Engineering. Also this notation is gaining acceptance through common usage in Oracle texts and in tools such as Visio, PowerDesigner, Toad Data Modeler, OmniGraffle and Dia.
Three symbols are used to represent cardinality:

the ring represents "zero"
the dash represents "one"
the crow's foot represents "more" or "many"

These symbols are used in pairs to represent the four types of cardinality that an entity may have in a relationship.

ring and dash → zero or one

dash and dash → exactly one

ring and crow's foot → zero or more

dash and crow's foot → one or more These are notations of a side in the ER diagrams.
You can see an example of the crow's foot notation in the diagram to the above.
In the diagram, the following facts are detailed:

An Artist can perform zero or more Songs

A Song is performed by exactly one Artist (Alt. note: this isn't a good example of cardinality in the real world, since multiple artists may collaborate on one song. This is an example of over-specifying.) Crow's foot notation has the following benefits:

Clarity in identifying the many, or child, side of the relationship, using the crow's foot.

Concise notation for identifying mandatory relationship, using a perpendicular bar, or an optional relation, using an open circle.Further Information

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