By C. J. Date
Perspectives are digital tables. that suggests they need to be updatable, simply as "real" or base tables are. in truth, view updatability isn’t simply fascinating, it’s an important, for useful purposes in addition to theoretical ones. yet view updating has continuously been a arguable subject. Ever because the relational version first seemed, there was frequent skepticism to whether (in normal) view updating is even possible.
In stark distinction to this traditional knowledge, this ebook exhibits how perspectives, similar to base tables, can regularly be up-to-date (so lengthy because the updates don’t violate any integrity constraints). extra often, it exhibits how updating constantly should paintings, whether the objective is a base desk or a view. The proposed scheme is a hundred% in step with the relational version, yet fairly varied from the way in which updating works in SQL items today.
This booklet can:
- Help database items enhance sooner or later
- Help with a "roll your own" implementation, absent such product advancements
- Make you conscious of the an important position of predicates and constraints
- Show you the way relational items are rather speculated to behave
- Anyone with a qualified curiosity within the relational version, relational expertise, or database platforms commonly can take advantage of this book.
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Extra info for View Updating and Relational Theory
I begin with the trite observation that a view is a relvar—a virtual relvar, to be precise, or in other words a relvar that “looks and feels” just like a real, or base, relvar but (unlike a real or base relvar) doesn’t exist independently of other relvars; rather, it’s defined in terms of, or derived from, such other relvars. Here’s a definition: Definition: A view is a derived, virtual relvar. The value of view V at time T is the result of evaluating a certain relational expression (the view defining expression) at that time T.
Activities of users at terminals and ... application programs should remain unaffected when the internal representation of data is changed and even when some aspects of the external representation are changed. The first sentence here says we want physical data independence; the second says we want logical data independence as well. 5 Physical data independence means we can change the way the data is physically stored and accessed without having to make changes in the way the data is perceived by the user; analogously, logical data independence means we can change the way the data is logically stored and accessed, again without having to make changes in the way the data is perceived by the user.
Note: I remind you from the preface that throughout this book I use “Database Design and Relational Theory” as an abbreviated form of reference to my book Database Design and Relational Theory: Normal Forms and All That Jazz (O’Reilly, 2012). The Technical Context / Chapter 2 23 You can think of a predicate as a truth valued function. Like all functions, it has a set of parameters; it returns a result when it’s invoked; and (because it’s truth valued) that result is either TRUE or FALSE. In the case of the predicate for relvar S, for example, the parameters are SNO, SNAME, STATUS, and CITY (corresponding of course to the attributes of the relvar), and they stand for values of the applicable types (CHAR, CHAR, INTEGER, and CHAR, respectively).
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