that4loopgirl

The features of the E – R includes specialization, generalization, higher level & lower level entity sets, attribute inheritance, and aggregation. 1. Specialization An entity set may have subgrouping of entities that are different from other entity set in some form. These entity grouping can be represented using E – R model. The process of describing subgrouping within an entity set is known as Specialization . An entity set may be specialized by more than one distinguish feature. For example, consider an entity set named person , having attributes name , street , and city . The person can be classified as: customer or employee . A set of attributes that includes all the attributes of entity person are described by each of these person type. For example, customer entities may be further described by attribute customer_id , and the employee entities can be further described by attributes employee_id, employee_salary. Consider another example, suppose bank decides to d

Individual entities are distinct and the difference among them is expressed through attributes. The values of the attributes should be such that it uniquely identifies the values of the entities. In other words, no two entities are allowed to have same value for all the attributes. Hence a Key is used to identify a set of attributes, relationships, and thus distinguish entities and relationships from each other. 1. Entity Set A superkey is a set of one or more attributes that allows us to uniquely identify an entity in entity set. For example, the customer_id attribute of entity set customer distinguishes one customer from another. Hence, customer_id is a superkey. The customer_name attribute of entity customer cannot be superkey because, two customers can have same name. Another example, the combination of customer_id and customer_name is also a superkey for entity customer. The superkey which does not have any subset is known as candidate key (or primary key) . For examp