Tables, Columns and Domains - Supertypes and Subtypes

In RDBMS environments, the relation between a real-life entity (such as a person) and its attributes (such as age and gender) is thought of in terms of tables (one for each entity) with one or more columns (one for each attribute). A number of the entities in the database will be quite similar in many respects while perhaps only slightly different in others. For these entities the question arises whether to:

·	Define one table with many optional columns.

·	Define a different table for each entity.

·	Define a common core of attributes (called supertypes) with variable additions (called subtypes).

USoft Developer supports all three methods, but recommends that you use the third one. For example, you could have a Person table containing names and addresses, and use subtypes to specify what type of job each person has, as shown below.

Example of supertypes and subtypes