E R model

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1. E-R Model Advantages and Disadvantages of E-R Model

2. Basic Techniques There are two techniques used for the purpose of data base designing from the system requirements. These are:• Top down Approach known as Entity-Relationship Modeling • Bottom Up approach known as Normalization.

3. Continue…… we will focus on top down approach of designing database. It is a graphical technique, which is used to convert the requirement of the system to graphical representation, so that it can become well understandable. It also provides the framework for designing of database. The Entity-Relationship (ER) model was originally proposed by Peter in 1976 as a way to unify the network and relational database views. Simply stated, the ER model is a conceptual data model that views the real world as entities and relationships. A basic component of the model is the Entity-Relationship diagram, which is used to visually represent data objects. For the database designer, the utility of the ER model is:  

4. Continue….. • It maps well to the relational model. The constructs used in the ER model can easily be transformed into relational tables. • It is simple and easy to understand with a minimum of training. Therefore, the model can be used by the database designer to communicate the design to the end user. • In addition, the model can be used as a design plan by the database developer to implement a data model in specific database management software.

5. What is ER-Model? In 1967 Peter Chen developed the ER Model. It is a high level data model used for developing the conceptual design of the database. ER diagram helps designers understand and specify the desired components of database and the relationship among them. It must be flexible enough so that it can be used and understood in practically any environment where information is modelled

6. Elements Of E-R Model There are three basis components of E-R model. These are (i) Entity (ii) Relation (iii) Attribute The mixture of above mentioned elements can create a effective E-R Diagram.

7. ENTITY It is a real world item / concept that can exist on it’s own. It may be an object with physical existence ( person, house) or it may be an object with conceptual existence (company ,job, university course)

8. ATTRIBUTE Attributes are the properties that describe the entities. Attribute names are enclosed by ovals and connected to their entities by single line. Set of attribute values of a given attribute is the value set or domain EMPLOYEE Ssn Salary Bdate

9. RELATIONSHIP When attribute of an entity refers to another entity type there exists relationship Ex: attribute ‘Manager’ of DEPARTMENT refers to another employee who manages the department. Attribute ‘Controlling-dep’t refers to another department that controls the project. In ER diagram these references must be represented as relationship and not attributes .

10. Advantages Following are advantages of an E-R Model: • Straightforward relation representation: Having designed an E-R diagram for a database application, the relational representation of the database model becomes relatively straightforward. • Easy conversion for E-R to other data model: Conversion from E-R diagram to a network or hierarchical data model can· easily be accomplished. • Graphical representation for better understanding: An E-R model gives graphical and diagrammatical representation of various entities, its attributes and relationships between entities. This is turn helps in the clear understanding of the data structure and in minimizing redundancy and other problems

11. Disadvantages Following are disadvantages of an E-R Model: • No industry standard for notation: There is no industry standard notation for developing an E-R diagram. • Popular for high-level design: The E-R data model is especially popular for high level.