T-SQL 数字位位数求和

数字 123456， 求和 =1+2+3+4+5+6 =21

declare @i int
set @i=123456
select 
	sum(substring(ltrim(@i),number,1)*1)
from 
	master..spt_values 
where 
	type='p' and number between 1 and len(@i)