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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33.5. Watch the IN Predicate
The IN predicate is really shorthand for a series of ORed equality tests. There are two forms: either an explicit list of values is given, or a subquery is used to make such a list of values.
The database engine has no statistics about the relative frequency of the values in a list of constants, so it will assume that the list is in the order in which the values are to be used. People like to order lists alphabetically or by magnitude, but it would be better to order the list from most frequently occurring values to least frequently occurring. It is also pointless to have duplicate values in the constant list, since the predicate will return TRUE if it matches the first duplicate it finds and will never get to the ...
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