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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19.4. Boolean Expressions in an RDBMS
Given the usual “hangar and pilots” schema, we want to create and store queries that involve Boolean expressions such as “Find the pilots who can fly a Piper Cub and also an F-14 or F-17 Fighter.” The trick is to put the expression into the disjunctive canonical form. In English that means a bunch of ANDed predicates that are then ORed together. Any Boolean function can be expressed this way. This form is canonical when each Boolean variable appears exactly once in each term. When all variables are not required to appear in every term, the form is called a disjunctive normal form. The algorithm to convert any Boolean expression into disjunctive canonical form is a bit complicated, but can be found in a good ...
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