Data models,Relational model

 Relational model

This model represents the database as a collection of tables.Each table is given a unique name and contains number of rows and columns  of  data. A row in a table gives the relational among the set of data values. Therefore a table can be viewed as a collection of relations as in mathemetics. Hence a table is also called as a relation and this model of data representaion is called relational model. The figure given below shows the structure of a relational database model for a polytechnic college with three relations or tables. they are,

• staff
• Admission
• Pay

Fields or attributes or columns

The table column headings are called attributes or fields. Each row of data in the table is called tuple or record.

Each tuple or record contains same number of  fields or columns.

Advantages

1.  Simplicity

            Since data are stored in a table as rows and cloumns, it is easy to design, add or delete data, change data and change data format.

2. Structural independence

            The change in the structure of the database dose not affect the data access.

3. Flexibility

        

        Different tables can be linked very easily.

4. Accurate

        The relation between different tabels can be calculated accurately using mathematical operations.

5 Security

        Security can be enforced easily by keeping confidential data in a separate table.

Disadvantages

        1. Relational database systems  need more powerful hardware components and data storege devices.

         2.  it is not possible to store special data such as spatial data, multimedia data, unstrauctured data and digital data.

        3. if the number of tables in a database increases performance degrades.

Networking model

    This model represents the database as a graph. each node is represent as a record. the nodes are connected according to a relationship. the relationship between nodes (records) are many to many(n.n)

that is a record can have more then one parent.

     

    Example

        1. An employee working in two departments

            jmr   hod             d2 it

            

            rks   prof             d4 e&c

    2. Consider the relational data model given below customer

                Cid               Cname            City

                A11               vikas                chaibasa        

                B23             vijay                   hatgamariya

                b27            Babu                    maduria    

        product

            pid            Pname                    Rate                Stock

            p1            cd                            3                    1000

            c2             ram                          250                500

            m1            monitor                    2500               25

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