Joe Celko's SQL for Smarties, 3rd Edition

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 ...