July 2010
Intermediate to advanced
840 pages
16h 33m
English
Content preview from Joe Celko's SQL for Smarties, 3rd Edition
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Chapter 26. Set Operations
BY SET OPERATIONS, I mean union, intersection, and set differences, where the sets in SQL are tables. These are the basic operators used in elementary set theory, which has been taught in the United States public school systems for decades. Since the relational model is based on sets, you would expect that SQL would have had a good variety of set operators from the start. However, this was not the case. Standard SQL has added the basic set operators, but they are still not common in actual products.
There is another problem in SQL that you did not have in high school set theory. SQL tables are multisets (also called bags), which means that, unlike sets, they allow duplicate elements (rows or tuples). Dr. Codd’s relational ...
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