7. ENTITY RELATIONSHIP MODELLING
Non-key attributes are attributes that are not part of any key. Generally, most attributes are simply descriptive, and fall into this category. Determining key and non-key attributes is an important modeling exercise, one that requires careful consideration. Consider an Employee entity type that has attributes for first name, last name, birth date; these attributes would serve to describe an employee but would not serve to uniquely identify employees.
People may join an organization and their name is not likely unique for the organization; we expect many people in a large organization to have the same first name, same last name, and even the same combination of first and last name. Names cannot usually be used as a key.
However, names chosen for entities such as departments in an organization could be keys because of the way the company would choose department names – they wouldn’t give two different departments the same name.
1) Consider a library and the fact that books are loaned out to library members. Dates could be used several times: for the date a book was borrowed, the date the book was returned, and the due date for a book. Consider an entity type Loan that has attributes book identifier, member identifier, date borrowed, date due, date returned. What combination of attributes would be a key? Which attributes are key attributes? Which attributes are non-key attributes
2) A birthdate attribute would appear for many entity types – for example students, employees, children. What is a birthdate likely to be: key or non-key?