TAILIEUCHUNG - An Introduction to Database Systems 8Ed - C J Date - Solutions Manual Episode 1 Part 3

Tham khảo tài liệu 'an introduction to database systems 8ed - c j date - solutions manual episode 1 part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | and what the primary and foreign keys are it isn t so important to know exactly what the sample data values are . Mention the fact that Fig. is repeated inside the back cover of the book for ease of subsequent reference. Answers to Exercises As usual some of the following definitions elaborate slightly on those given in the body of the chapter. The term automatic navigation refers to the fact that in a relational system the process of navigating around the stored data in order to implement user requests is performed automatically by the system not manually by the user. A base relvar also known as a real relvar is a relvar that has independent or autonomous existence. More precisely it s a relvar that isn t a derived relvar . . It s not necessarily the same thing as a stored relvar. The catalog is a set of system relvars whose purpose is to contain descriptors regarding the various objects that are of interest to the system itself such as base relvars views indexes users integrity constraints security constraints and so on. The term closure of relational operations refers to the fact that a the output from any relational operation is the same kind of object as the input they re all relations and so b the output from one operation can become input to another. Closure implies that we can write nested relationvalued expressions. Note We stress the point that when we say that the output from each operation is another relation we are talking from a conceptual point of view. We don t necessarily mean to imply that the system actually has to materialize the result of every individual operation in its entirety. In fact of course the system tries very hard not to if such materialization is logically unnecessary see the brief discussion of pipelined evaluation in the body of the chapter . Commit is the operation that signals successful end-of-transaction. Any updates made to the database by the transaction in question are now made permanent and become visible .

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