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Generalisation and Specialisation

GENERALISATION
Consider the account entity set with attributes accno and balance. Each account can be classified as SAVINGS ACCOUNT or CURRENT ACCOUNT. Each of these is described by a set of attributes which include all the attributes of the account entity set PLUS some additional attributes , SAVINGS ACCOUNT entities are described by the attribute INTEREST RATE ,while CURRENT ACCOUNT are described by the attribute OVER DRAFT AMOUNT. There are similarities between the current account entity set and the saving account entity set in the sense that they have several attributes in common . This commonality can be expressed by generalization. Generalization is a containment relationship that exist between a higher level entity set and one or more lower level entity sets .Here the ACCOUNT is the higher level entity and SAVINGS ACCOUNT & CURRENT ACCOUNT are LOWERLEVEL ENTITY SETS. In an ER diagram generalization is represented by a triangular component labeled “ISA" it stands for "is a ". For example Savings Account "is an " account .The attributes of higher level entity sets are inherited by the lower level entity sets.




ISA realtionship between ACCOUNT - SAVINGS ACCOUNT & CURRENT ACCOUNT

There are two methods for transforming an ER diagram, which includes generalization, to a tabular form.
FIRST METHOD - Create a table for the higher level entity set. For each lower level entity set create a table which includes columns for each of the attributes of that entity set, plus columns for each of the attributes of the primary key of the higher level entity set.
Then we will have three tables for the above ER diagram .
a). Account with columns accno & balance
b). Savings account with columns accno & interest rate
c). Current account with columns accno. & overdraft amount
SECOND METHOD - Do not create a table for higher level entity set. For each lower level entity set create a table which includes columns for each of the attributes of that entity set, plus columns for each of the attributes of the higher level entity set.
Then we will have two tables for the above ER diagram .
a). Savings account with columns accno, balance & interest rate
b). Current Account with columns accno, balance & overdraft amount.

AGGREGATION
Consider a data base describing information about employees who work on a particular project and uses a number of different machines in there work ,we will obtain the following ER diagram using our basic ER model. We may think that the relation ship sets WORKS & USES can be combined into one single relationship set. But they cannot be combined. Because the ternary relationship obtained by combining these cannot represent a relationship between a manager, (who doesn’t uses any machine) and a project.
Aggregation is an abstraction through which relationships are treated as higher level entities. In this case we consider the relationship set work and the entity sets employee and project as a higher level entity set WORKS. Such an entity is treated in the same manner as any other entity set using the diagram WORKS.






Aggregation :: EMPLOYEE - PROJECT - MACHINE relationship

Using the normal transformation procedure, we will get table for employee, project, machine , works and uses. The table for relationship set uses will include columns for each attribute in the primary key of the entity set machine and the relation ship works. ie. { empcode, pcode, mcode}.

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