假如有个表：
field1 field2 field3
1 2.3 4.5
2 2.3 3.4
4 3.4 4.5
6 4.5 5.6
7 5.6 6.7
想显示为:
field1 field2 field3 field4
1 2.3 4.5 0
2 2.3 3.4 -1.1
4 3.4 4.5 -2.2
6 4.5 5.6 - 3.3
7 5.6 6.7 -4.4
field4的计算规则是，第一条为0，
后面的就等于上一条的field4+本条的field2-本条的field3
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select field1,field2,field3,
isnull((select sum(field2)-sum(field3)
from mytable t1
where t1.field1<=t2.field1
and t1.field1<>(select min(field1) from mytable)),0)
from mytable t2
order by field1