create table t1
(id int, price decimal(5,1), [sz-status] char(1))

insert into t1
select 1, 12, 's' union all  
select 2, 3, 'z' union all    
select 3, 2.5, 'z' union all  
select 4, 4.6, 'z' union all  
select 5, 10, 's'


select a.id,a.price,a.[sz-status],
(select sum(case b.[sz-status] 
            when 's' then b.price
            when 'z' then -1*b.price end)
 from t1 b where b.id<=a.id) '结余'
from t1 a

/*
id          price        sz-status   结余
----------- ------------ --------- ---------
1           12.0             s       12.0
2           3.0              z       9.0
3           2.5              z       6.5
4           4.6              z       1.9
5           10.0             s       11.9

(5 row(s) affected)
*/