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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21.3. AVG() Functions
AVG([ALL] <value expression>) returns the average of the values in the value expression set. An empty set returns an empty result set. A set of all NULLs will become an empty set. Remember that in general, AVG(x) is not the same as (SUM(x)/COUNT(*)); the SUM(x) function has thrown away the NULLs, but the COUNT(*) has not.
Likewise, AVG(DISTINCT <value expression>) returns the average of the distinct known values in the <value expression> set. Applying this function to a key or a unique column is the same as the using AVG(<value expression>) function.
Remember that in general AVG(DISTINCT x) is not the same as AVG(x) or (SUM(DISTINCT x)/COUNT(*)). The SUM(DISTINCT x) function has thrown away the duplicate values and NULL ...
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