关于日期统计问题(高手帮忙啊）

[雪狼1234567]

如下：
userid M1, M2, M3, M4
11 10:20:00 null null null
11 null 11:20:00 null null
11 null null 23:20:00 null
11 null null null 20:20:00
我想要的结果是
UserID M1 M2 M3 M4
11 10:20:00 11:20:00 23:20:00 20:20:00
请高手帮忙，想用sum函数结果提示字符串类型不能用，

[zicxc]

select userid,max(M1) as M1,max(M2) as M2,Max(M3) as M2,max(M4) as M4
from tablename
group by userid

[DontWorry]

sELECT USERID, MAX(M1) AS M1, MAX(M2) AS M2, MAX(M3) AS M3, MAX(M4) AS M4
from T_TABLE

[DontWorry]

加 句 GROUP BY

[victorycyz]

select userid,
max(isnull(m1,'00:00:00')) as m1,
max(isnull(m2,'00:00:00')) as m2,
max(isnull(m3,'00:00:00')) as m3,
max(isnull(m4,'00:00:00')) as m4
from tablename
group by userid