Intoduction to Entity Relationship Diagram
Entity Relationship Diagram represents the object relationship pairs in graphical forms thus we can say that the primary goal of E-R diagram s is to represent data objects along with their relationship. The various components of data modelling described above forms the primary components of entity relationship diagram.
ER model for data uses three features to describe data:
Entities: which specify distinct real world items in an application?
Relationships: connecting different entities and representing meaningful dependencies between them.
Attributes: which specify various properties of entities and relations involved in a system.
In ER diagram data objects are represented by labelled rectangle. All its attributes are represented by labelled ellipse connected to data objects by straight line. There are two ways in which relationships are represented in an ER diagram. One way is to use a labelled straight line. Another also knows as conventional way is to use a labelled diamond connected to dependent data objects by straight line.
These two ways are shown in fig 4.6, where an employee works in a company. The attributes of two entities are also shown.
In addition to basis ER notations an analyst can represent data object type hierarchy. For example data object mobile phone can be categorised as GSM color display and camera with color display which can further be categorised. It is shown in fig4.7.
ER diagram provides the most convenient way examining data within the context of software.
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