create table tb(time varchar(20))
insert into tb values('0000:00:05.84')
insert into tb values(null)
insert into tb values('00:10:05.84')
insert into tb values('0000:22:22.22')
insert into tb values('0000:11:12.09')
go


select sum(datediff(ss,'1900-01-01' , cast(substring(time,charindex(':',time) + 1 , len(time)) as datetime))) From tb
/*
            
----------- 
157440

（所影响的行数为 1 行）
*/

select avg(datediff(ss,'1900-01-01' , cast(substring(time,charindex(':',time) + 1 , len(time)) as datetime))) From tb
/*
            
----------- 
39360

（所影响的行数为 1 行）
*/

drop table tb

------解决方案--------------------
SQL code

declare @t table (time varchar(13))
insert into @t
select '0000:00:05.84' union all
select null union all
select '00:10:05.84' union all
select '0000:22:22.22' union all
select '0000:11:12.09'

select  sum(isnull(datediff(ss, '1900-01-01',
                            cast(right(time, 11) as datetime)), 0))
from    @t
/*
2624
*/

------解决方案--------------------

SQL code

declare @t table (time varchar(13))
insert into @t
select '0000:00:05.84' union all
select null union all
select '00:10:05.84' union all
select '0000:22:22.22' union all
select '0000:11:12.09'

DECLARE @i INT 
select  @i=sum(isnull(datediff(ms, '1900-01-01',
                            cast(right(time, 11) as datetime)), 0))
from    @t

SELECT LTRIM(@i/1000)+'.'+LTRIM(@i%1000)+'秒'
/*
2625.990秒
*/
SELECT LTRIM(@i/60000)+'分钟'+LTRIM(@i/60000)+'秒'+LTRIM(@i%1000)+'毫秒'
/*
43分钟43秒990毫秒
*/