在CASE语句中处理NULL

I have a mysql query:

SELECT CASE WHEN roomid = 1 THEN SUM(taxmoney) END END AS 'total1',
       CASE WHEN roomid = 2 THEN SUM(taxmoney) END AS 'total2', 
       DATE_FORMAT(end_at,'%Y-%m-%d') AS 'date_time' FROM match_logs GROUP BY 
       DATE_FORMAT(end_at,'%Y-%m-%d')

And this is my result when executed it:

total1 || total2 || date_time
52040  || NULL   || 2015-03-31
89803  || NULL   || 2015-04-01
45250  || 4658   || 2015-04-02

I want to change NULL in total2 column as 0. What should i do?

Thanks all!

Use this command for total2:

 CASE WHEN roomid = 2 THEN IFNULL(SUM(taxmoney),'0') END AS 'total2', 

The complete command:

SELECT CASE WHEN roomid = 1 THEN SUM(taxmoney) END AS 'total1',
       CASE WHEN roomid = 2 THEN IFNULL(SUM(taxmoney),'0') END AS 'total2', 
       DATE_FORMAT(end_at,'%Y-%m-%d') AS 'date_time' FROM match_logs GROUP BY 
       DATE_FORMAT(end_at,'%Y-%m-%d')