What is subtype discriminator example?
What is subtype discriminator example?
For example, in a university environment, a person may be an employee or a student or both. Because an employee may also be a student, STUDENT and EMPLOYEE are overlapping subtypes of the supertype PERSON, just as PROFESSOR and ADMINISTRATOR are overlapping subtypes of the supertype EMPLOYEE.
What is a subtype discriminator?
A subtype discriminator is an attribute in the supertype entity that determines to which subtype the supertype occurrence is related. Disjount and overlapping constraints. Disjoint subtypes: Contain a unique subset of the supertype entity set. Each entity instance of the supertype may only exist as one of the subtypes.
What purpose subtype discriminator serves?
The purpose of the subtype discriminator is to help you avoid having to write subqueries or joins just to find information such as the names of all AARP members.
What are supertypes and subtypes?
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.
What is discriminator in ER diagram?
The discriminator (or partial key) of a weak entity set is the set of attributes that distinguishes among all the entities of a weak entity set on one particular strong entity.
What is a discriminator in ERD?
Subtype Discriminators A subtype discriminator is an attribute of the supertype that indicates an entity’s subtype. The attribute’s values are what determine the target subtype. Disjoint subtypes – simple attributes that must have alternative values to indicate any possible subtypes.
Can subtypes have primary keys?
All relations representing entity types in the same hierarchy have the same primary key. The primary key of a subtype relation will also be a foreign key that references its supertype relation.
What do double diamonds represent in an ER diagram?
Explanation: Diamonds represent relationship sets in an ER diagram. Relationship sets define how two entity sets are related in a database. Explanation: The double diamonds represent the relationship sets linked to weak entity sets.
What is weak entity in ER diagram?
A weak entity is a type of entity which doesn’t have its key attribute. It can be identified uniquely by considering the primary key of another entity. For that, weak entity sets need to have participation. In above ER Diagram examples, “Trans No” is a discriminator within a group of transactions in an ATM.
What is the use of EER diagram?
Enhanced entity-relationship (EER) diagrams are basically an expanded upon version of ER diagrams. EER models are helpful tools for designing databases with high-level models. With their enhanced features, you can plan databases more thoroughly by delving into the properties and constraints with more precision.
What is the difference between ERD and EERD?
ER vs EER Diagram ER diagram is a visual representation of data based on ER model, and it describes how entities are related to each other in the database. EER diagram is a visual representation of data, based on EER model that is an extension of the original entity-relationship (ER) model.
Is it good practice to include a subtype in a supertype?
At the conceptual modeling stage, it is good practice to include an OTHER subtype to make sure that your subtypes are exhaustive — that you are handling every instance of the supertype. Any entity can be subtyped by making up a rule that subdivides the instances into groups.
How are supertypes and subtypes mutually exclusive in SQL?
Exhaustive: Every instance of the supertype is also an instance of one of the subtypes. All subtypes are listed without omission. Mutually Exclusive: Each instance of a supertype is an instance of only one possible subtype.
How are nonoverlapping subtypes implemented in entity set?
Known as nonoverlapping subtypes Implementation is based on the value of the subtype discriminator attribute in the supertype Overlapping subtypes: Contain nonunique subsets of the supertype entity set Implementation requires the use of one discriminator attribute for each subtype
What makes an exam a subtype of a quiz?
EXAM is a supertype of QUIZ, MIDTERM, and FINAL. The subtypes have several attributes in common. These common attributes are listed at the supertype level. The same applies to relationships. Subtypes inherit all attributes and relationships of the supertype entity.