How to Build a Business Rules Engine

Account Number and Description of Item, attributes that only apply to purchases.

Likewise, Cash Advance has the unique attributes Issuing Bank ABA Number and

Cash Advance Fee Amount.

Figure 3.4 shows the category symbol—the circle with the two lines under-

neath—which indicates we are dealing with a subtype relationship. There has to be

an attribute in the supertype to distinguish between the different subtypes. This

attribute is called a category discriminator, and in Figure 3.4 it is Transaction Type ID.

It is listed next to the category symbol. This attribute comes from the entity Trans-

action Type where it is the primary key, and this entity is ...