What is subclass in ER diagram?
The overlap rule forces a subclass (also known as a supertype instance) to have overlapping sets of entities. The total specialization rule demands that every entity in the superclass belong to some subclass. Just as with a regular ERD, total specialization is symbolized with a double line connection between entities.
Can a subtype have another subtype?
In general, a supertype will have two or more subtypes. However, a subtype is (directly) a subtype of only one supertype. Thus, the use of supertypes and subtypes in ERDs resembles the use of “single inheritance” between classes, in object-oriented programming.
What is supertype and subtype in database?
A supertype is a generic entity type that has a relationship with one or more subtypes. A subtype is a sub-grouping of the entities in an entity type that is meaningful to the organization and that shares common attributes or relationships distinct from other subgroups. Subtypes inherit all supertype attributes.
What are the types of ER diagram?
There are two kinds of ER diagrams: conceptual and physical. Conceptual diagram models can be used as the foundation for logical data models or to form commonality relationships between ER models as a basis for data-model integration.
What is class and subclass?
The derived class (the class that is derived from another class) is called a subclass. The class from which its derived is called the superclass. Definition: A subclass is a class that derives from another class. A subclass inherits state and behavior from all of its ancestors.
What is base and sub class?
Definitions: A class that is derived from another class is called a subclass (also a derived class, extended class, or child class). The class from which the subclass is derived is called a superclass (also a base class or a parent class).
Can a subtype have a relationship?
Inclusive subtype relationship (in IE, when an attribute in the subtype entity can derive its value from one or more subtype entities). Exclusive subtype relationship (in IE, when an attribute in the subtype entity can derive its value from only one subtype entity).
Do subtypes have primary keys?
Subtypes are subgroups of the supertype entity and have unique attributes, but they will be different from each subtype. Supertypes and Subtypes are parent and child entities respectively and the primary keys of supertype and subtype are always identical.
What is an example of subtype?
Subtypes are a subset of features in a feature class, or objects in a table, that share the same attributes. They are used as a method to categorize your data. For example, the streets in a city streets feature class could be categorized into three subtypes: local streets, collector streets, and arterial streets.
Does a subtype need a primary key?
A person does not have to exist as one of the subtypes. Note the foreign keys: Student has a foreign key referencing SubjectArea. If a row exists in Employee or Student then a corresponding row must exist in Person.
What is double line in ER diagram?
Participation Constraints Total Participation − Each entity is involved in the relationship. Total participation is represented by double lines. Partial participation − Not all entities are involved in the relationship. Partial participation is represented by single lines.