Conceptual entity relationship model

Conceptual, Logical and Physical Data Model

conceptual entity relationship model

y Conceptual design: (ER Model is used at this stage.) – What are the entities and relationships in the enterprise? – What information about these entities and. The world that we live in can be abstracted out as a simple model which can be used to solve many existing problems. The primary concept of. This definition explains the meaning of entity relationship diagram, also known as A logical data model, which is more detailed than a conceptual data model.

Entity–relationship model - Wikipedia

We use this reference to simply call all the entities at once of a selected type. Each entity in a given entity set should have a unique way of identifying it. These are called "Keys. Given below are the thee different types of keys, that can be used.

conceptual entity relationship model

This is followed by an example. One or more attributes that can be used together to uniquely identify that entity, from the given entity set. A minimal super key, that can be used to uniquely identify an entity. A super key can be a set of attributes used to uniquely identify an entity.

Removing an attribute from it, can still have a chance of making it a super key.

ER Model in hindi (Simple & Easy Explain)

But in candidate keys, we can;t remove any attributes, because that will kill the uniqueness of the entity. There can be multiple candidate keys for a given entity. A primary key, is one of the candidate keys, chosen by the designer to uniquely represent the entity. There can only be one Primary Key. Each candidate key, is a candidate for the primary key, of a given entity.

conceptual entity relationship model

What is a Relationship A relationship basically explains how one entity is related to another. Object-role modeling Crow's foot notation[ edit ] Crow's foot notation, the beginning of which dates back to an article by Gordon Everest[12] is used in Barker's notationStructured Systems Analysis and Design Method SSADM and information technology engineering.

Crow's foot diagrams represent entities as boxes, and relationships as lines between the boxes. Different shapes at the ends of these lines represent the relative cardinality of the relationship. Crow's foot notation was used in the consultancy practice CACI. With this notation, relationships cannot have attributes. Where necessary, relationships are promoted to entities in their own right: Model usability issues[ edit ] You can help by adding to it.

Entity–relationship model

February In using a modeled database, users can encounter two well known issues where the returned results mean something other than the results assumed by the query author. The first is the 'fan trap'. It occurs with a master table that links to multiple tables in a one-to-many relationship. The issue derives its name from the way the model looks when it's drawn in an entity—relationship diagram: This type of model looks similar to a star schemaa type of model used in data warehouses.

When trying to calculate sums over aggregates using standard SQL over the master table, unexpected and incorrect results. The solution is to either adjust the model or the SQL.

conceptual entity relationship model

This issue occurs mostly in databases for decision support systems, and software that queries such systems sometimes includes specific methods for handling this issue. 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.

Conceptual, Logical and Physical Data Model

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.

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.