Tham khảo tài liệu 'an introduction to database systems 8ed - c j date - solutions manual episode 1 part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | P WHERE COLOR COLOR Purple P PER SP S P Again stress the usefulness of WITH in breaking complex expressions down into step-at-a-time ones. Also stress the fact that using WITH does not sacrifice nonprocedurality. Note The discussion of projection in Section includes the following question Why can t any attribute be mentioned more than once in the attribute name commalist The answer of course is that the commalist is supposed to denote a set of attributes and attribute names in the result must therefore be unique. The discussion under Example includes the following text Begin quote The purpose of the condition SA SB is twofold It eliminates pairs of supplier numbers of the form x x . It guarantees that the pairs x y and y x won t both appear. End quote The example might thus be used if desired to introduce the concepts of Reflexivity A binary relation R A B is reflexive if and only if A and B are of the same type and the tuple A x B x appears in R for all applicable values x of that common type. Symmetry A binary relation R A B is reflexive if and only if A and B are of the same type and whenever the tuple A x B y appears in R then the tuple A y B x also appears in R . See . reference for further discussion. What s the Algebra for Dispel the popular misconception that the algebra or the calculus is just for queries. Note in particular that the algebra or calculus is fundamentally required in order to be able to express integrity constraints which is why Chapters 7 and 8 precede Chapter 9. Copyright c 2003 C. J. Date page Regarding relational completeness The point is worth making that once Codd had defined this notion of linguistic power it really became incumbent on the designer of any database language either to ensure that the language in question was at least that powerful or to have a really good justification for not doing so. And there really isn t any good justification . This fact is a cogent criticism of several nonrelational .
