mysql - 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:

tab    
+---------------+-----+
| descr         | num |
+---------------+-----+
| hello there   | 5   |
| hi there      | 10  |
| hello         | 10  |
| hi there!     | 15  |
+---------------+-----+

This query:

SELECT sum(num) AS val FROM tab WHERE descr LIKE "%hello%";

should, and does, return 15. However:

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

should return 0, but does return null.

Can someone explain if this is possible?

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.