In object-oriented programming (OOP) and both database theory and design, 'isa' refers to a special type relationship which can exist between objects. There are two relatively common instances where objects may have an 'isa' relationship:
If one or more objects (children) can have an isa relationship with another object (parent) if the if the children inherit from the parent.
If one or more objects implement a common interface or extend an abstract class, they will have an is-a relationship with whatever they are implementing/extending.
Database Theory/design
The ‘isa’ relationship relates to subclasses inheriting attributes from the superclass.
The method of identifying subsets of existing entity sets is called specialization. This corresponds with the notion of subclass and class inheritance in object-oriented design, where it is represented by class hierarchies (See superclass).
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