Böcker av Chris Date

Keys and foreign keys play a crucial role in relational databases-keys identify the objects of interest, and foreign keys knit those objects together. The basic idea couldn't be simpler. As so often, however, the devil is in the detail ... The fact is, these concepts aren't quite as straightforward as they might seem on first acquaintance-or, at least, such would appear to be the case, if the literature is anything to go by. In this one of a kind book, noted database author C. J. Date traces the somewhat checkered history of the key and foreign key concepts, shedding some light on what turns out to be, on occasion, a surprisingly murky subject and explaining in detail what proper support should look like in true relational products. Topics covered include a detailed look at the pertinent theory; a critical review of the historical development of these ideas; and a couple of important case studies, one having to do with the SQL standard and one with the IBM DB2 product family. No serious database professional can afford to be without this book.

A set in mathematics is just a collection of elements; an example is the set of natural numbers {1, 2, 3, ...}. Simplifying somewhat, the theory of sets can be regarded as the foundation on which the whole of mathematics is built; and the founder of set theory is the German logician and mathematician Georg Cantor (1845¿1918). However, the aspect of Cantor's work that's most widely known-or most controversial, at any rate-isn't so much set theory in general, but rather those parts of that theory that have to do with infinite sets in particular. Cantor claimed among other things that the infinite set of real numbers contains strictly more elements than the infinite set of natural numbers. From this result, he concluded that there's more than one kind of infinity; in fact, he claimed that there are an infinite number of different infinities, or transfinite numbers. (He also believed these results had been communicated to him by God.) The aim of this book is to explain and investigate these claims of Cantor's in depth (and question them, where appropriate). It's not a textbook, though; instead, it's a popular account-it tells a story-and the target audience is interested lay readers, not mathematicians or logicians. What little mathematics is needed to understand the story is explained in the book itself.

Some things seem so obvious that they don''t need to be spelled out in detail. Or do they? In computing, at least (and probably in any discipline where accuracy and precision are important), it can be quite dangerous just to assume that some given concept is "obvious," and indeed universally understood. Serious mistakes can happen that way! The first part of this book discusses features of the database field-equality, assignment, naming-where just such an assumption seems to have been made, and it describes some of the unfortunate mistakes that have occurred as a consequence. It also explains how and why the features in question aren''t quite as obvious as they might seem, and it offers some advice on how to work around the problems caused by assumptions to the contrary. Other parts of the book also deal with database issues where devoting some preliminary effort to spelling out exactly what the issues in question entailed could have led to much better interfaces and much more carefully designed languages. The issues discussed include redundancy and indeterminacy; persistence, encapsulation, and decapsulation; the ACID properties of transactions; and types vs. units of measure. Finally, the book also contains a detailed deconstruction of, and response to, various recent pronouncements from the database literature, all of them having to do with relational technology. Once again, the opinions expressed in those pronouncements might seem "obvious" to some people (to the writers at least, presumably), but the fact remains that they''re misleading at best, and in most cases just flat out wrong.