Phương pháp và ngữ pháp cho mối quan hệ Khái quát / Chuyên ngànhChúng ta cần phải xem lại bước 6 trong ER Phương pháp thiết kế để bao gồm các khả năng của mối quan hệ khái quát hóa / chuyên môn. Các phiên bản trước của bước 6: Bước 6: Nhà nước chính xác bản chất của mối quan hệ trong tiếng Anh có cấu trúc từ tất cả các bên | Methodology and Grammar for Generalization Specialization Relationships We need to revisit step 6 in the ER Design Methodology to cover the possibility of generalization specialization relationships. The previous version of step 6 was Step 6 State the exact nature of the relationships in structured English from all sides. For example if a relationship is A B 1 M then there is a relationship from A to B 1 to Many and from B back to A Many to 1. For ternary and higher-order n ary relationships state the relationship in structured English being careful to mention all entities for the n ary relation. State the structural constraints as they exist. We add the following sentence to step 6 For specialization generalization relationships state the relationship in structured English being careful to mention all entities subclasses or specializations . State the structural constraints as they exist. The grammar that we propose for specializations generalizations relationships is similar to that we used in weak relationships. We add to the grammar to include the participation overlapping disjointness constraints The grammatical description for weak entities was For each weak entity we do not assume that any attribute will be unique enough to identify individual entities. Because the weak entity does not have a candidate key each weak entity will be indentified by key s belonging to the strong entity. In the case of our athlete a first attempt to describe the subclass identified by a superclass becomes For each ATHLETE in a SPORT we do not assume that any sport attribute will be unique enough to identify individual SPORT entities. Because the SPORT does not have a candidate key each SPORT will be identified by inheriting key s belonging to ATHLETE. So a more complete EER diagram grammatical pattern would say For each specialization we do not assume that any attribute will be unique enough to identify individual entities. Because the specialization does not have a candidate key
