一个SQL算法，多谢大家了！

--测试环境
if object_id('[tb]') is not null drop table [tb]
go   
create table [tb]([客户] varchar(1),[单据类型] varchar(4),[余额] numeric(10,1),[单据日期] datetime,账龄 int)
insert [tb]
select 'A','应收','1292301','2010-1-31 00:00','121' union all
select 'A','应收','1139292','2010-2-28 00:00','93' union all
select 'A','应收','1152316.8','2010-3-23 00:00','70' union all
select 'A','应收','78148.8','2010-3-23 00:00','70' union all
select 'A','应收','1230465.6','2010-4-28 00:00','34' union all
select 'A','应收','1204416','2010-5-25 00:00','7' union all
select 'A','收款','-1204416','2010-2-28 00:00','93' union all
select 'A','收款','-1139292','2010-3-31 00:00','62' union all
select 'A','收款','-1230465.6','2010-4-30 00:00','32' union all
select 'A','收款','-1230465.6','2010-5-31 00:00','1' union all
select 'B','应收','196668','2010-1-31 00:00','121' union all
select 'B','应收','8866','2010-2-25 00:00','96' union all
select 'B','应收','154677.6','2010-2-28 00:00','93' union all
select 'B','应收','508741.2','2010-3-23 00:00','70' union all
select 'B','应收','453157.2','2010-4-28 00:00','34' union all
select 'B','应收','411469.2','2010-5-25 00:00','7' union all
select 'B','收款','-196668','2010-3-31 00:00','62' union all
select 'B','收款','-154677.6','2010-4-30 00:00','32' union all
select 'B','收款','-8866','2010-4-30 00:00','32' union all
select 'B','收款','-508741.2','2010-5-31 00:00','1' union all
select 'C','应收','506268','2010-1-21 00:00','131' union all
select 'C','应收','536491.2','2010-2-25 00:00','96' union all
select 'C','应收','633080','2010-3-23 00:00','70' union all
select 'C','应收','426927.2','2010-4-19 00:00','43' union all
select 'C','应收','913399.2','2010-5-17 00:00','15' union all
select 'C','应收','959968.8','2009-12-30 00:00','153' union all
select 'C','应收','584511.2','2009-9-30 00:00','244' union all
select 'C','应收','974943.2','2009-4-30 00:00','397' union all
select 'C','应收','45332.82','2009-1-23 00:00','494' 
go

select 客户,
sum(余额)as 应收余额,
0 as 账期内本币金额,
case    when    sum(case when 单据类型='应收'and 账龄 > 30 then 余额 else 0 end)+
                sum(case when 单据类型='收款' then 余额 else 0 end)>=0
        then    sum(case when 单据类型='应收'and 账龄 between 1 and 31 then 余额 else 0 end)
        else    case when    sum(case when 单据类型='应收'and 账龄 > 31 then 余额 else 0 end)+
                            sum(case when 单据类型='收款' then 余额 else 0 end) +
                            sum(case when 单据类型='应收'and 账龄 between 1 and 30 then 余额 else 0 end)<0 
                    then 0 
                    else    sum(case when 单据类型='应收'and 账龄 > 30 then 余额 else 0 end)+
                            sum(case when 单据类型='收款' then 余额 else 0 end) +
                            sum(case when 单据类型='应收'and 账龄 between 1 and 30 then 余额 else 0 end) 
                end
end as [1-30天余额],
case    when    sum(case when 单据类型='应收'and 账龄 > 60 then 余额 else 0 end)+
                sum(case when 单据类型='收款' then 余额 else 0 end)>=0
        then    sum(case when 单据类型='应收'and 账龄 between 31 and 60 then 余额 else 0 end)
        else    case when    sum(case when 单据类型='应收'and 账龄 > 60 then 余额 else 0 end)+
                            sum(case when 单据类型='收款' then 余额 else 0 end) +
                            sum(case when 单据类型='应收'and 账龄 between 31 and 60 then 余额 else 0 end)<0 
                    then 0 
                    else    sum(case when 单据类型='应收'and 账龄 > 60 then 余额 else 0 end)+
                            sum(case when 单据类型='收款' then 余额 else 0 end) +
                            sum(case when 单据类型='应收'and 账龄 between 31 and 60 then 余额 else 0 end) 
                end
end as[31-60天余额],
case    when    sum(case when 单据类型='应收'and 账龄 > 90 then 余额 else 0 end)+
                sum(case when 单据类型='收款' then 余额 else 0 end)>=0
        then    sum(case when 单据类型='应收'and 账龄 between 61 and 90 then 余额 else 0 end)
        else    case when    sum(case when 单据类型='应收'and 账龄 > 90 then 余额 else 0 end)+
                            sum(case when 单据类型='收款' then 余额 else 0 end) +
                            sum(case when 单据类型='应收'and 账龄 between 61 and 90 then 余额 else 0 end)<0 
                    then 0 
                    else    sum(case when 单据类型='应收'and 账龄 > 90 then 余额 else 0 end)+
                            sum(case when 单据类型='收款' then 余额 else 0 end) +
                            sum(case when 单据类型='应收'and 账龄 between 61 and 90 then 余额 else 0 end) 
                end
end as[61-90天余额],
case    when    sum(case when 单据类型='应收'and 账龄 > 180 then 余额 else 0 end)+
                sum(case when 单据类型='收款' then 余额 else 0 end)>=0
        then    sum(case when 单据类型='应收'and 账龄 between 91 and 180 then 余额 else 0 end)
        else    case when    sum(case when 单据类型='应收'and 账龄 > 180 then 余额 else 0 end)+
                            sum(case when 单据类型='收款' then 余额 else 0 end) +
                            sum(case when 单据类型='应收'and 账龄 between 91 and 180 then 余额 else 0 end)<0 
                    then 0 
                    else    sum(case when 单据类型='应收'and 账龄 > 180 then 余额 else 0 end)+
                            sum(case when 单据类型='收款' then 余额 else 0 end) +
                            sum(case when 单据类型='应收'and 账龄 between 91 and 180 then 余额 else 0 end) 
                end
end as[91-180天余额],
case    when    sum(case when 单据类型='应收'and 账龄 > 270 then 余额 else 0 end)+
                sum(case when 单据类型='收款' then 余额 else 0 end)>=0
        then    sum(case when 单据类型='应收'and 账龄 between 181 and 270 then 余额 else 0 end)
        else    case when    sum(case when 单据类型='应收'and 账龄 > 270 then 余额 else 0 end)+
                            sum(case when 单据类型='收款' then 余额 else 0 end) +
                            sum(case when 单据类型='应收'and 账龄 between 181 and 270 then 余额 else 0 end)<0 
                    then 0 
                    else    sum(case when 单据类型='应收'and 账龄 > 270 then 余额 else 0 end)+
                            sum(case when 单据类型='收款' then 余额 else 0 end) +
                            sum(case when 单据类型='应收'and 账龄 between 181 and 270 then 余额 else 0 end) 
                end
end as[181-270天余额],
case    when    sum(case when 单据类型='应收'and 账龄 > 360 then 余额 else 0 end)+
                sum(case when 单据类型='收款' then 余额 else 0 end)>=0
        then    sum(case when 单据类型='应收'and 账龄 between 271 and 360 then 余额 else 0 end)
        else    case when    sum(case when 单据类型='应收'and 账龄 > 360 then 余额 else 0 end)+
                            sum(case when 单据类型='收款' then 余额 else 0 end) +
                            sum(case when 单据类型='应收'and 账龄 between 271 and 360 then 余额 else 0 end)<0 
                    then 0 
                    else    sum(case when 单据类型='应收'and 账龄 > 360 then 余额 else 0 end)+
                            sum(case when 单据类型='收款' then 余额 else 0 end) +
                            sum(case when 单据类型='应收'and 账龄 between 271 and 360 then 余额 else 0 end) 
                end
end as[271-360天余额],
case    when    sum(case when 单据类型='应收'and 账龄 > 720 then 余额 else 0 end)+
                sum(case when 单据类型='收款' then 余额 else 0 end)>=0
        then    sum(case when 单据类型='应收'and 账龄 between 361 and 720 then 余额 else 0 end)
        else    case when    sum(case when 单据类型='应收'and 账龄 > 720 then 余额 else 0 end)+
                            sum(case when 单据类型='收款' then 余额 else 0 end) +
                            sum(case when 单据类型='应收'and 账龄 between 361 and 720 then 余额 else 0 end)<0 
                    then 0 
                    else    sum(case when 单据类型='应收'and 账龄 > 360 then 余额 else 0 end)+
                            sum(case when 单据类型='收款' then 余额 else 0 end) +
                            sum(case when 单据类型='应收'and 账龄 between 361 and 720 then 余额 else 0 end) 
                end
end as[361-720天余额],
case    when    sum(case when 单据类型='应收'and 账龄 > 720 then 余额 else 0 end)+
                sum(case when 单据类型='收款' then 余额 else 0 end)>=0
        then    sum(case when 单据类型='应收'and 账龄 > 720 then 余额 else 0 end)+
                sum(case when 单据类型='收款' then 余额 else 0 end)
        else    0
end as[721天以上余额]
from tb
group by 客户
--结果
/*
客户   应收余额          账期内本币金额     1-30天余额       31-60天余额      61-90天余额      91-180天余额     181-270天余额    271-360天余额    361-720天余额    721天以上余额
----  ------------ -----------  ------------  ------------  ------------  ------------  ------------  ------------  ------------  ------------
A    1292301.0     0           1204416.0     87885.0       0.0           0.0           0.0           0.0           0.0           0.0
B    864626.4      0           411469.2      453157.2      0.0           0.0           0.0           0.0           0.0           0.0
C    5580921.6     0           913399.2      426927.2      633080.0      2002728.0     584511.2      0.0           1020276.0     0.0

(3 行受影响)

*/