Tham khảo tài liệu 'an introduction to database systems 8ed - c j date - solutions manual episode 2 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Chapter 20 T y p e I n h e r i t a n c e Principal Sections Type hierarchies Polymorphism and substitutability Variables and assignments S by C Comparisons Operators versions and signatures Is a circle an ellipse S by C revisited SQL facilities General Remarks Note the opening remarks This chapter relies heavily on material first discussed in Chapter 5. If you originally gave that chapter a once over lightly reading therefore you might want to go back and revisit it now before studying the present chapter in any depth. To be more specific a clear understanding of the following is prerequisite What a type is reviewed in Section . The crucial distinction between values and variables see Section . Note Object-based discussions typically fall foul of this distinction since they re often unclear as to whether an object is a value or a variable or both or neither. This failure seems to be at the root of the famous infamous debate as to whether . a circle is an ellipse. See Section . The crucial distinction between read-only and update operators again see Section . Note The point is that read-only operators apply to values possibly values that are the current values of variables while update operators apply to variables. Copyright c 2003 C. J. Date page Every type has all of the following among other things An associated type constraint which defines the set of legal values of the type in question At least one declared possible representation together with a corresponding selector operator and a corresponding set of THE_ operators or logical equivalents of same and operators Certain type testing operators to be discussed in Section these operators might be unnecessary in the absence of inheritance support also TREAT DOWN to be discussed in Section All of these bullet items except the last are also explained in Chapter 5. The following preliminaries from Section are also important Values are typed . have actual most specific types
