As mentioned, finding the right level of specialization is a significant database design challenge. In reviewing the logical design as proposed in Figure 7-6, the database design team noticed something: The only difference among the Sole Proprietorship, Partnership, and Corporation subtypes is in the way that the names of key people in those types of companies appear as attributes. Moreover, the use of two nearly identical attributes for the names of the co-owners in the Partnership subtype could be considered a repeating attribute, and therefore a first normal form violation.

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INDIVIDUAL CUSTOMER FIRST NAME MIDDLE INITIAL CUSTOMER NUMBER (FK) SOLE PROPRIETORSHIP CUSTOMER NUMBER (FK) CUSTOMER NUMBER CUSTOMER TYPE ADDRESS OWNER NAME CUSTOMER CITY STATE ZIP CODE PHONE LAST NAME DATE OF BIRTH ANNUAL HOUSEHOLD INCOME FIGURE 7-6 Customer subclasses CUSTOMER TYPE COMMERCIAL CUSTOMER CUSTOMER NUMBER (FK) COMPANY NAME TAX IDENTIFICATION NUMBER ANNUAL GROSS REVENUE COMPANY TYPE PARTNERSHIP CUSTOMER NUMBER (FK) PARTNER NAME 1 PARTNER NAME 2 COMPANY TYPE CORPORATION CUSTOMER NUMBER (FK) CEO NAME BOARD CHAIR NAME INDIVIDUAL CUSTOMER FIRST NAME MIDDLE INITIAL CUSTOMER NUMBER (FK) SOLE PROPRIETORSHIP CUSTOMER NUMBER (FK) CUSTOMER NUMBER CUSTOMER TYPE ADDRESS OWNER NAME CUSTOMER CITY STATE ZIP CODE PHONE LAST NAME DATE OF BIRTH ANNUAL HOUSEHOLD INCOME FIGURE 7-6 Customer subclasses CUSTOMER TYPE COMMERCIAL CUSTOMER CUSTOMER NUMBER (FK) COMPANY NAME TAX IDENTIFICATION NUMBER ANNUAL GROSS REVENUE COMPANY TYPE PARTNERSHIP CUSTOMER NUMBER (FK) PARTNER NAME 1 PARTNER NAME 2 COMPANY TYPE CORPORATION CUSTOMER NUMBER (FK) CEO NAME BOARD CHAIR NAME