Tuy nhiên, nó là bất hợp pháp để bán, kiếm tiền từ nó trong bất kỳ cách nào. Không ai có thể thêm các trang, loại bỏ các trang, hoặc thay đổi nó. Hoàn thành cấp giấy phép: tổ chức phải trả tiền để sử dụng cuốn sách này | 166 CHAPTER 4 THE ENHANCED E-R MODEL AND BUSINESS RULES In Figure 4rl7 supertype subtype hierarchies are used extensively. For example J in the PARTY ROLE entity type the hierarchy is as many as four levels deep . PARTY ROLE to PERSON ROLE to CONTACT to CUSTOMER CONTACT . Attributes can be located with any entity type in the hierarchy . PARTY has the identifier of PARTY ID means identifier PERSON has three optional attributes o means optional and ORGANIZATION has a required attribute means required . Relationships can be between entity types anywhere in the hierarchy. For example any EVENT is in the state of an EVENT STATUS TYPE a subtype whereas any EVENT is within the context of a PARTY RELATIONSHIP a supertype. As stated previously packaged data models are not meant to be exactly right straight out of the box for a given organization they are meant to be customized. To ị be the most generalized such models have certain properties before they are customized for a given situation 1. Relationships are connected to the highest level entity type in a hierarchy that makes sense. Relationships can be renamed eliminated added and moved as need be for the organization. 2. Strong entities almost always have M N relationships between them . EVENT and PARTY so at least one and sometimes many associative entities are used. Consequently all relationships are 1 M and there is an entity type in which to store intersection data. Intersection data are often dates showing over what span of time the relationship was valid. Thus the packaged data 1 model is designed to allow tracking of relationships over time. Recall this is a common issue that was discussed with Figure 3-20. 1 M relationships are j optional at least on the many side . the dotted line next to EVENT for I the involving relationship signifies an EVENT may involve an EVENT I ROLE as is done with Oracle Designer . j 3. Although not clear on this diagram all supertype subtype relationships fol- low the total
