E-R model

ER model stands for an Entity-Relationship model. It is a high-level data model which  is used to define the data elements and relationship between the elements in the system.

It develops a conceptual design for the database. this model is  a very simple and easy to design 

    Components of E-R model:--

1....Entity:--

An entity may be any object, class, person or place. In the ER diagram, an entity can be represented as rectangles.    

       Weak-Entity:--An entity that depends on another entity called a weak entity. The weak entity doesn't contain any key attribute of its own.

       Strong-Entity:---An entity having key attributes.

2.Attributes:--Attributes are defined as characterstics of an entity .Eclipse is used for representing attributes 

      . simple/composite attributes:---simple attributes can't be further classified.

       As composite attributes can be further classified .Composite attributes are as collection of simple attributes.

   . Single valued/multivalued attributes:---Single valued attributes contains one value ,whereas multivalued contain more than one value in a single field.      

         Stored/derived ----Stored attribute of an entity ,whereas derived attributes are the one which are derived from the stored one.

       Complex attributes:--Collection of both multivalued and composite attributes are called complex one.

Relationship:----

Simply defines the relation between two entities. Diamond symbol represents the relationship among entities.

       One-to-one:---When only one instance of an entity is associated with the relationship

      One-to-many relationship:-----When only one instance of the entity on the left, and more than one instance of an entity on the right associates with the relationship

Many-to-one relationship:---When more than one instance of the entity on the left, and only one instance of an entity on the right associates with the relationship 

Many-to-many relationship:---When more than one instance of the entity on the left, and more than one instance of an entity on the right associates with the relationship

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