Question

Could you expound in database modeling, how subclass and superclass are used? Maybe you can use these terms(cardinality constraint , total participation constraint, weak entity type, partial key, IS-A relationship, specialization and generalization, specific (or local) attribute, specific relationship, aggregation)

Answer 1

Specialized class are often called as subclass while generalized class are called superclass, probably inspired by object oriented programming. A sub-class is best understood by “IS-A analysis”. Following statements hopefully makes some sense to your mind “Technician IS-A Employee”, “Laptop IS-A Computer”.

An entity is specialized type/class of other entity. For example, Technician is special Employee in a university system Faculty is special class of Employee. We call this phenomenon as generalization/specialization. In the example here Employee is generalized entity class while Technician and Faculty are specialized class of Employee.

Example – This example instance of “sub-class” relationships. Here we have four sets employee: Secretary, Technician, and Engineer. Employee is super-class of rest three set of individual sub-class is subset of Employee set.

• An entity belonging to a sub-class is related with some super-class entity. For instance emp no 1001 is a secretary, and his typing speed is 68. Emp no 1009 is engineer (sub-class) and her trade is “Electrical”, so forth.
• Sub-class entity “inherits” all attributes of super-class; for example employee 1001 will have attributes eno, name, salary, and typing speed.

Constraints – There are two types of constraints on “Sub-class” relationship.

• Total or Partial – A sub-classing relationship is total if every super-class entity is to be associated with some sub-class entity, otherwise partial. Sub-class “job type based employee category” is partial sub-classing – not necessary every employee is one of (secretary, engineer, and technician), i.e. union of these three types is proper subset of all employees. Whereas other sub-classing “Salaried Employee AND Hourly Employee” is total; union of entities from sub-classes is equal to total employee set, i.e. every employee necessarily has to be one of them.
• Overlapped or Disjoint – If an entity from super-set can be related (can occur) in multiple sub-class sets, then it is overlapped sub-classing, otherwise disjoint. Both the examples: job-type based and salaries/hourly employee sub-classing are disjoint.