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If I run a query such as:
SELECT COUNT(*) as num FROM table WHERE x = 'y'
Will it always return a result, even when the query doesn’t match any record? Or do I need to validate and make sure a row is returned as the result?
Yes, because it’s an aggregate and returns zero.
Unless you add GROUP BY in which case no result because there is no group…
MAX/SUM etc would return NULL unless you add GROUP BY then no rows.
Only COUNT returns a number for no results
Edit, a bit late: SUM would return NULL like MAX
Edit, May 2013: this applies to all main RDBMS. I guess as per the ANSI standard