.8 Cũng như các cơ hội làm nổi bật để khái quát mối quan hệ, vòng cung độc quyền có thể đề nghị siêu kiểu thực thể lớp tiềm năng. Trong hình 4,13, chúng ta được nhắc nhở để supertype Công ty, cá nhân, đối tác, và Cơ quan Chính phủ, có lẽ để người nộp thuế (Hình 4,14). Chúng tôi thấy rằng chúng ta sử dụng vòng cung độc quyền khá thường xuyên trong | 196 Chapter Ó Primary Keys and Identity When to Use Structured Keys The rale for using structured keys is straightforward you can include a foreign key in a primary key only if it represents a mandatory ÍIOÍI-transferable4 I dalionstiip. The relationship needstobe mandatory because an optional relationship wouldmc-anstiaitomnioyt wouldhsvysnurieaSuefor theforesgn key hence tlicpriinnsylscy fot mloae rowtwouldbe problemsofneha in c rimahkuneolumnsaredltcusswomnuerien . The reaionfor the nentransSernbliity mnynsi iy - so eCvieus. Ton problem with traestorabUo retalirnaVsps tr lbsu tltu vnSue el tire fnioign key will needou enawgewyee la t erb liyn tt t rrensfsrrcd ks onow cwnee For c oycclisti i cdt omonc departmant toanothec the valueonDyhartmrr rie theSorniev Toy is part oSlheprimurykey the n uo hcve ectiangcm vnSuso4 thu primue key and aviolarronof ons thiscyvmp DepiertmrnHD should iier form sartof ton primarykey ur dmptoyee. Another tvavol fonkieg ntthtsrituhtsou istlrou ifwestricdy follow the rule that primarckirwyrrou sconvytcOan CoswerOoulsi 1 thce sti iictiiied keys can be- strt-dr. nnfonck . reeslracturedkey implemeeis rOs rate diae deprodentontity mstands-i cannot lie tranttb ns s ni from onhewnelansitnto enoihc-t . emote- dclmlcd example using the notation for nontransferayiiity introdoked in rhctidn eiSi6. Tee nteck Holyinn entity class hno 1 01x1110111. ro eoth Stock and Client. Sn ttus ss terms 1. An instance of Stock Holding cannot exist without corresponding instances of stock and ctiest. 2. An instav en S ror t Hsldmgeannot be transferred to a different stock or client By contrast the relationship from client to Investment Advisor is optional and Iiso 1 -. .c oe I . the tersmessriiter that 1. We can hold information about a client who does not have an investment adviser.
