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NULL is not zero!

Some time ago I wrote a post about how COUNT(*) and COUNT(column) are semantically different things (link). Such queries may return different results if the column counted has NULLs in it. And the difference comes from that NULL is not a value, it’s rather a state which says “value unknown” or “no value entered”.
So, you better understand how NULLs interact with your SQL constructs if you call yourself a DBA or a database developer ;-)