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

Check the MySQL documentation for IFNULL.

SELECT SUM(IFNULL(num, 0)) as val FROM tab WHERE descr LIKE "%greetings%";

Of course, this assumes your num field is nullable and doesn't have a default value. Another possible solution would be to set a default of 0 for the num field which should solve the issue you're having.