generalization specialization relational modeling code example

Example: what is specialization and generalization in dbms

Generalization : It is a bottom – up approach in which multiple lower
				 level entities are combined to form a single higher
                 level entity. It is usually used to find common 
                 attributes among entities to form a generalized entity. 

Specialization : It is a top – down approach in which a higher level
				 entity is divided into multiple lower – level entities.
                 In addition to sharing the attributes of the higher 
                 level entity, these lower – level entities have specific
                 attributes of their own. It is usually used to find
                 subsets of an entity that has a few different, 
                 or additional attributes. 

Aggregation : It is a process in which a single 
			  entity alone is not able to make sense in a relationship. 
              So, a relationship of 2 entities acts as 1 entity. 
              It is a way of composing different abstractions together 
              in defining a class.