Analyzing Data › Defining Entity Subtypes and Partitioning

Defining Entity Subtypes and Partitioning

You find that entities belonging to the same entity type have differing characteristics. This situation requires the definition of entity subtypes and partitioning.

For example, consider this expanded attribute list for the entity type CUSTOMER:

• Number
• Name
• Address
• Phone Number
• Status
• Credit Rating
• Outstanding Orders Value
• Nationality (Foreign Or Domestic)
• Country Code
• Import License Number
• Currency
• Tax ID Number
• State of Incorporation
• Type (Commercial Or Government)
• Government Agency Name
• Interested Politician Name

Given this set of attributes, assume the following attributes to be true:

• The attributes: Country Code, Import License Number, and Currency have values only for FOREIGN CUSTOMERs.
• The attributes: Tax ID Number and State of Incorporation have values only for DOMESTIC CUSTOMERs.
• The attributes: Government Agency Name and Interested Politician Name are valid for either FOREIGN CUSTOMERs or DOMESTIC CUSTOMERs, but only when the CUSTOMER is a GOVERNMENT CUSTOMER, not a COMMERCIAL CUSTOMER.

You could record these facts as a set of integrity constraints on CUSTOMER. However, you would handle this situation by defining types.