ENTITY  RELATIONSHIP  MODEL

  Entity  Relationship  Model : 

Entity  Relationship  Model  ( E-R )  data  model  is  based  on  a  perception  of  a  real  world  that  consists  of  a  set  of  basic  objects  called  entities  and  of  relationships  among  these  objects.  It  was  developed  to  facilitate  database  design  by  allowing  the  specification  of  an  enterprise scheme, which  represents  the  overall  logical  structure  of  a  database.

 Basic  Concepts :

There  are  three  basic  notations  that  the  E-R  data  model  involves – entity  sets, attributes  and  relationship  sets.

Entity Relationship (ER) modeling

• is a design tool
• is a graphical representation of the database system
• provides a high-level conceptual data model
• supports the user’s perception of the data
• is DBMS and hardware independent
• had many variants
• is composed of entities, attributes, and relationships

Entity Sets :

• An entity is any object in the system that we want to model and store information about Individual objects are called entities
• Groups of the same type of objects are called entity types or entity sets
• Entities are represented by rectangles (either with round or square corners)
• There are two types of entities; weak and strong entity types.
• Sometimes it is useful to try out various examples of entities from an ER model. One reason for this is to confirm the correct cardinality and optionality of a relationship.

Attributes :

• All the data relating to an entity is held in its attributes.
• An attribute is a property of an entity.
• Each attribute can have any value from its domain.
• Each entity within an entity type:
  • May have any number of attributes.
  • Can have different attribute values than that in any other entity.
  • Have the same number of attributes.
• Attributes can be-  1. simple or composite ,  2. single-valued or multi-valued
• Attributes can be shown on ER models
• They appear inside ovals and are attached to their entity.
• Note that entity types can have a large number of attributes… If all are shown then the diagrams would be confusing. Only show an attribute if it adds information to the ER diagram, or clarifies a point.

Relationships  :

• A relationship type is a meaningful association between entity types
• A relationship is an association of entities where the association includes one entity from each participating entity type.
• Relationship types are represented on the ER diagram by a series of lines.
• As always, there are many notations in use today…
• In the original Chen notation, the relationship is placed inside a diamond.

Problems with ER Models :

There are several problems that may arise when designing a conceptual data model. These are known as connection traps.

There are two main types of connection traps:

1. fan traps
2. chasm traps

 Constructing an ER model :

Before beginning to draw the ER model, read the requirements specification carefully. Document any assumptions you need to make.

1. Identify entities – list all potential entity types. These are the object of interest in the system. It is better to put too many entities in at this stage and them discard them later if necessary.
2. Remove duplicate entities – Ensure that they really separate entity types or just two names for the same thing.
   • Also do not include the system as an entity type
   • e.g. if modeling a library, the entity types might be books, borrowers, etc.
   • The library is the system, thus should not be an entity type.
3. List the attributes of each entity (all properties to describe the entity which are relevant to the application).
   • Ensure that the entity types are really needed.
   • are any of them just attributes of another entity type?
   • if so keep them as attributes and cross them off the entity list.
   • Do not have attributes of one entity as attributes of another entity!
4. Mark the primary keys.
   • Which attributes uniquely identify instances of that entity type?
   • This may not be possible for some weak entities.
5. Define the relationships
   • Examine each entity type to see its relationship to the others.
6. Describe the cardinality and optionality of the relationships
   • Examine the constraints between participating entities.
7. Remove redundant relationships
   • Examine the ER model for redundant relationships.

ER modeling is an iterative process, so draw several versions, refining each one until you are happy with it. Note that there is no one right answer to the problem, but some solutions are better than others!

Mapping  Constraints :

An  E-R  scheme  may  define  certain  constraints  to  which  the  contents  of  a  database  must  conform.

a)  Mapping  Cardinalities:  Mapping  Cardinalities  express  the  number  of  entities  to  which  another  entity  can  be  associated  via  a  relationship. Different  form  of  mapping  cardinalities  are –

One-to-One:

An  entity  in  A  is  associated  with  at  most  one  entity  in  B  and  an  entity  in  B  is  associated  with  at  most  one  entity  in  A.

For  example ,  A  customer  is  associated  with  at  most  one account.  An  account  is  associated  with  at  most  one  customer.

Figure : One-to-one from customer to account

One-to-Many:

An  entity  in  A  is  associated  with  any  number  of  entities  in  B  and  an  entity  in  B,  however,  can  be  associated  with  at  most  one  entity  in  A

For  example , in  the  one-to-many  relationship  an  account  is  associated  with  at  most  one customer.  A  customer  is  associated  with several accounts.

Many-to-one:

An  entity  in  A  is  associated  with at  most  one  entity  in  B  and  an  entity  in  B,  however,  can  be  associated  with any  number  of  entities  in

For  example , in  the  many-to-one  relationship  a  customer  is  associated  with several accounts , an  account  is  associated  with  at  most  one customer.

Figure : Many-to-one from customer to account

Many-to-many:

An  entity  in  A  is  associated  with any  number of  entities   in  B  and  an  entity  in  B  is    associated  with any  number  of  entities  in  A.

4.5    Keys :

• A key is a data item that allows us to uniquely identify individual occurrences or an entity type.
• A candidate key is an attribute or set of attributes that uniquely identifies individual occurrences or an entity type.
• An entity type may have one or more possible candidate keys, the one which is selected is known as the primary key.
• A composite key is a candidate key that consists of two or more attributes
• The name of each primary key attribute is underlined.

Several  types  of  keys  are –

Primary Key:

A  primary  key  is  a  field  or  combination  of  fields  that  uniquely  identify  a  record  in  a  table, so  that  an  individual  record  can  be  located  without confusion.

Foreign keys :

A foreign key is an attribute (or group of attributes) that is the primary key to another relation.

• Roughly, each foreign key represents a relationship between two entity types.
• They are added to relations as we go through the mapping process.
• They allow the relations to be linked together.
• A relation can have several foreign keys.
• It will generally have a foreign key from each table that it is related to.
• Foreign keys are usually shown in italics or with a wiggly underline.

The Entity Relationship Diagram :

We can express the overall logical structure of a database graphically with an E-R diagram.

Its components are:

• rectangles representing entity sets.
• ellipses representing attributes.
• diamonds representing relationship sets.
• lines linking attributes to entity sets and entity sets to relationship sets.

  Database  Of  This  Site :

Table  Name :  Registration  ( user  information )

Table  Name :  Yarn  Information

Table  Name :  Sacking  and  Hessian ( bag  & cloth)  Information

E-R  Diagram  According  To  Our  Website: