How to select sum -or- 0 if no records exist?

Question

I need to write a query that returns the sum of all values that meet a certain criteria, but the query needs to return 0 if no rows are found, rather than null. For example

Answer 1

How about:

SELECT COALESCE(sum(num), 0) AS val FROM tab WHERE descr LIKE "%greetings%";

The COALESCE function basically says "return the first parameter, unless it's null in which case return the second parameter" - It's quite handy in these scenarios.