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

This works:

SELECT IF(SUM(num) IS NULL, 0, SUM(num)) AS val FROM tab WHERE descr LIKE "%whatever%";

IF() takes three parameters: (1) A statement, (2) the value to apply if the statement is true, and (3) the value to apply if the statement is false.