There is concept of "Strong" and "Weak" entities in RDBMS. Strong Entity: Attributes of which are able to form Primery key i.e. To form a key which can uniquely identify its member. Weak Entity: Attributes of which are unable to form Primery key i.e. To form a key which can uniquely identify its member. Eg. Consider following relations in of Computer Engg Department context relation 1: Class { Division Name, Class Room No., Class Teacher } it is Strong Entity as any member can be uniquely identified by its Division Name, hence it is primery key. And relation 1: Student { Roll No., Name, Attendance } it is Weak Entity as any member can not be uniquely identified by any attribute or combination of attributes. As Roll no. Of students in different divisions can be same. But if we combine "Roll no." with "Division Name" then any member of Student relation can be identified uniquely. And hence for Student relation "Roll no." is Discriminator.
