What are the components of entity relationship data model

Entity Relationship Model in DBMS

what are the components of entity relationship data model

Relationships describe associations among data. Most relationships describe associations between two entities. when the basic data model components were . CS Home Data Modeling ERD Entities Relationships Attributes Example misjon.info Software Lab Assignment MySQL There are three components in ERD . E-R Diagrams in DBMS: Components, Symbols, And Notations E-R diagrams are used to represent E-R model in a database, which makes.

Each entity has attributes which are the properties of each entity. Attributes will be implemented as columns in the tables. Each attribute has a domain which specifies the set of possible values an attribute can have. For instance, the range of values for a telephone extension may be specified as a set of integer numbers between and An attributes domain is not displayed in ER diagrams, but is recorded in the data dictionary. Attributes are represented by ovals with lines connecting them to the entity.

Attributes can be of various types.

entity-relationship diagram (model)

A composite attribute can be subdivided into smaller parts. Attributes that cannot be subdivided are called simple attributes. First Name and Last Name are now simple attributes.

Most attributes have only a single value and as such are called single valued attributes.

what are the components of entity relationship data model

Multivalued attributes can have more than one value. For example, a Student could have more than one Certificate or a Department may have several Extensions.

What is Entity Relationship Diagram? Webopedia Definition

There are a number of diagramming conventions used to represent multivalued attributes. Some conventions implement the multivalued attribute as an oval with a double line around it. Others use a double bar to connect the attribute to the entity. The second issue is a 'chasm trap'. A chasm trap occurs when a model suggests the existence of a relationship between entity types, but the pathway does not exist between certain entity occurrences.

For example, a Building has one-or-more Rooms, that hold zero-or-more Computers. One would expect to be able to query the model to see all the Computers in the Building. However, Computers not currently assigned to a Room because they are under repair or somewhere else are not shown on the list.

Another relation between Building and Computers is needed to capture all the computers in the building.

E-R Diagrams in DBMS: Components, Symbols, And Notations

This last modelling issue is the result of a failure to capture all the relationships that exist in the real world in the model. See Entity-Relationship Modelling 2 for details. Entity—relationships and semantic modeling[ edit ] Semantic model[ edit ] A semantic model is a model of concepts, it is sometimes called a "platform independent model".

It is an intensional model. At the latest since Carnapit is well known that: The first part comprises the embedding of a concept in the world of concepts as a whole, i. The second part establishes the referential meaning of the concept, i.

what are the components of entity relationship data model

Extension model[ edit ] An extensional model is one that maps to the elements of a particular methodology or technology, and is thus a "platform specific model". The UML specification explicitly states that associations in class models are extensional and this is in fact self-evident by considering the extensive array of additional "adornments" provided by the specification over and above those provided by any of the prior candidate "semantic modelling languages".

It incorporates some of the important semantic information about the real world. Plato himself associates knowledge with the apprehension of unchanging Forms The forms, according to Socrates, are roughly speaking archetypes or abstract representations of the many types of things, and properties and their relationships to one another.

E-R Diagrams in DBMS: Components, Symbols, And Notations

Limitations[ edit ] ER assume information content that can readily be represented in a relational database. They describe only a relational structure for this information.

what are the components of entity relationship data model

They are inadequate for systems in which the information cannot readily be represented in relational form[ citation needed ], such as with semi-structured data. For many systems, possible changes to information contained are nontrivial and important enough to warrant explicit specification. An alternative is to model change separately, using a process modeling technique.

Additional techniques can be used for other aspects of systems.

Database Lesson #4 of 8 - Data Modeling and the ER Model