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I hear a lot about subtyping tables when designing a database, and I'm fully aware of the theory behind them. However, I have never actually seen table subtyping in action. How can you create subtypes of tables? I am using MS Access, and I'm looking for a way of doing it in SQL as well as through the GUI (Access 2003).
Cheers!
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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 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.
A subtype is a definition of a type based on a built-in type. Subtypes provide a layer of abstraction between variables and parameters and the data types that they use. This layer allows you to concentrate any changes to the data types in one location.
The entity subtype will store the data that is specific to the entity; that is, attributes that are unique the subtype.
An easy example would be to have a Person table with a primary key and some columns in that table. Now you can create another table called Student that has a foreign key to the person table (its supertype). Now the student table has some columns which the supertype doesn't have like GPA, Major, etc. But the name, last name and such would be in the parent table. You can always access the student name back in the Person table through the foreign key in the Student table.
Anyways, just remember the following:
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Subtypes of tables is a conceptual thing in EER diagrams. I haven't seen an RDBMS (excluding object-relational DBMSs) that supports it directly. They are usually implemented in either
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