How are subtype supertype relationships converted to relations?
Strategy is: Convert the supertype entity directly into a relation using only those attributes in the supertype – note the key. Convert each of the subtype entities into relations – also only with the attributes they contain. Copy the key for the supertype entity into each of the subtypes.
What type of relationship is established between a supertype and an subtype?
6. What type of relationship is established between a supertype and a subtype? Between the supertype and subtype set relation “one to one” or 1:1.
What are the conditions that must be met for using Supertype subtype relationships?
Two conditions that indicate a designer should consider using supertype/subtype relationships:
- There are attributes that apply to some (but not all) of the instances of an entity type.
- The instances of a subtype participate in the relationship unique to that subtype.
What the primary key and foreign key in relation to subtype and supertype?
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. Attributes of a supertype (except for the primary key) appear only in the relation that represents the supertype.
Can 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 attribute of the supertype which is used to differentiate the subtypes?
A subtype discriminator is the attribute in the supertype entity that is used to determine to which entity subtype the supertype occurrence is related. For any given supertype occurrence, the value of the subtype discriminator will determine which subtype the supertype occurrence is related to.
What is supertype and subtype in data modeling?
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 Supertype relationship?
How are supertypes related to subtypes in relational databases?
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. Attributes of a supertype (except for the primary key) appear only in the relation that represents the supertype.
What is the purpose of mapping supertypes and subtypes?
Mapping supertypes and subtypes makes sure that the right information gets stored with each type. This choice produces a single table for the implementation of the supertype entity and its subtypes. This is also called “single-table (or one-table) implementation.”
Can a supertype be modeled as an arc relationship?
A supertype entity and its subtypes can be modeled as an arc relationship. Here again is the original ERD with the supertype and subtypes. In this ERD, we have redrawn the CLOTHING supertype and its subtypes of SHIRT and SHOE as standalone entities… …with each one having mandatory 1:1 relationships with the supertype.
Which is the primary key of a subtype relation?
The primary key of a subtype relation will also be a foreign key that references its supertype relation. Attributes of a supertype (except for the primary key) appear only in the relation that represents the supertype. The library model maps to the following relational design: