DBMS 面试题与答案

A collection of conceptual tools for describing data, data relationships data semantics and constraints.

This model is based on collection of objects. An object contains values stored in instance variables with in the object. An object also contains bodies of code that operate on the object. These bodies of code are called methods. Objects that contain same types of values and the same methods are grouped together into classes. 

An entity set may not have sufficient attributes to form a primary key, and its primary key compromises of its partial key and primary key of its parent entity, then it is said to be Weak Entity set. 

It is a 'thing' in the real world with an independent existence.