Cooking with MySQL
On Lamp brings us:
Cooking with MySQL.
"It makes a lot of sense for aggregate functions to ignore NULL values. If they followed the usual SQL arithmetic rules, adding NULL to any other value would produce a NULL result. That would make aggregate functions really difficult to use because you'd have to filter out NULL values yourself every time you performed a summary to avoid getting a NULL result. Ugh. By ignoring NULL values, aggregate functions become a lot more convenient."
(Submitted by Noel Tue Nov 26, 2002 )
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