7. ENTITY RELATIONSHIP MODELLING
If an attribute’s value can be derived from the values of other attributes, then the attribute is derivable, and is said to be a derived attribute. For example, if we have an attribute for birth date then age is derivable. Derived attributes are shown with a dotted lined oval.
Sometimes an attribute of one entity type is derived from attributes from other entity types. Consider the attribute for the total of an Invoice. A value of InvTotal is derivable; it can be computed from invoice lines. Someone who implements a database and applications that access the database would need to decide whether the value of a derivable attribute should be computed when the entity is stored or updated versus computing the value (on-the-fly) when it is needed.
1) Consider an educational environment where the institution tracks the performance of each student. Often this is called the students overall average, or overall grade point average. Is such an attribute a derived attribute? How is its value determined?
2) Consider a library application that needs to keep track of books that have been borrowed. Suppose there is an entity type Loan that has attributes bookID, memberID, dateBorrowed and dateDue. Suppose the due date is always 2 weeks after the borrowed date. Show Loan and its attributes in an ERD.