我认为您需要使用recursive common table expression。
编辑：我还在下面添加了一个程序实现，而不是使用递归公用表表达式。我建议使用这种程序方法，因为我认为我所包含的递归 CTE 查询可能无法处理许多数据场景。
下面的查询为您提供的场景提供了正确答案，但您可能需要想出一些额外的复杂场景并查看是否存在任何错误。
例如，我有一种感觉，如果您有多个先前的订单与后来的订单重叠，则此查询可能会失败。
with CurrentOrders (UserId, SubscriptionDays, StartDate, EndDate) as
(
select
userid,
sum(subscriptiondays),
min(orderdate),
dateadd(day, sum(subscriptiondays), min(orderdate))
from #orders
where
#orders.orderdate <= @currentdate
-- start with the latest order(s)
and not exists (
select 1
from #orders o2
where
o2.userid = #orders.userid
and o2.orderdate <= @currentdate
and o2.orderdate > #orders.orderdate
)
group by
userid
union all
select
#orders.userid,
#orders.subscriptiondays,
#orders.orderdate,
dateadd(day, #orders.subscriptiondays, #orders.orderdate)
from #orders
-- join any overlapping orders
inner join CurrentOrders on
#orders.userid = CurrentOrders.UserId
and #orders.orderdate < CurrentOrders.StartDate
and dateadd(day, #orders.subscriptiondays, #orders.orderdate) > CurrentOrders.StartDate
)
select
UserId,
sum(SubscriptionDays) as TotalSubscriptionDays,
min(StartDate),
sum(SubscriptionDays) - datediff(day, min(StartDate), @currentdate) as RemainingSubscriptionDays
from CurrentOrders
group by
UserId
;
Philip 提到了对公用表表达式的递归限制的担忧。下面是一个使用表变量和 while 循环的程序替代方案，我相信它可以完成同样的事情。
虽然我已验证此替代代码确实有效，但至少对于提供的示例数据而言，我很高兴听到任何人对这种方法的 cmet。好主意？馊主意？有什么需要注意的吗？
declare @ModifiedRows int
declare @CurrentOrders table
(
UserId int not null,
SubscriptionDays int not null,
StartDate date not null,
EndDate date not null
)
insert into @CurrentOrders
select
userid,
sum(subscriptiondays),
min(orderdate),
min(dateadd(day, subscriptiondays, orderdate))
from #orders
where
#orders.orderdate <= @currentdate
-- start with the latest order(s)
and not exists (
select 1
from #orders o2
where
o2.userid = #orders.userid
and o2.orderdate <= @currentdate
-- there does not exist any other order that surpasses it
and dateadd(day, o2.subscriptiondays, o2.orderdate) > dateadd(day, #orders.subscriptiondays, #orders.orderdate)
)
group by
userid
set @ModifiedRows = @@ROWCOUNT
-- perform an extra update here in case there are any additional orders that were made after the start date but before the specified @currentdate
update co set
co.SubscriptionDays = co.SubscriptionDays + #orders.subscriptiondays
from @CurrentOrders co
inner join #orders on
#orders.userid = co.UserId
and #orders.orderdate <= @currentdate
and #orders.orderdate >= co.StartDate
and dateadd(day, #orders.subscriptiondays, #orders.orderdate) < co.EndDate
-- Keep attempting to update rows as long as rows were updated on the previous attempt
while(@ModifiedRows > 0)
begin
update co set
SubscriptionDays = co.SubscriptionDays + overlap.subscriptiondays,
StartDate = overlap.orderdate
from @CurrentOrders co
-- join any overlapping orders
inner join (
select
#orders.userid,
sum(#orders.subscriptiondays) as subscriptiondays,
min(orderdate) as orderdate
from #orders
inner join @CurrentOrders co2 on
#orders.userid = co2.UserId
and #orders.orderdate < co2.StartDate
and dateadd(day, #orders.subscriptiondays, #orders.orderdate) > co2.StartDate
group by
#orders.userid
) overlap on
overlap.userid = co.UserId
set @ModifiedRows = @@ROWCOUNT
end
select
UserId,
sum(SubscriptionDays) as TotalSubscriptionDays,
min(StartDate),
sum(SubscriptionDays) - datediff(day, min(StartDate), @currentdate) as RemainingSubscriptionDays
from @CurrentOrders
group by
UserId
EDIT2：我已经对上面的代码进行了一些调整以解决各种特殊情况，例如，如果恰好有两个用户的订单都在同一日期结束。
例如，将设置数据更改为以下会导致原始代码出现问题，我现在已经更正了：
insert into #orders
select 1, 2, 10, '2011-01-01'
union
select 2, 1, 10, '2011-01-10'
union
select 3, 1, 10, '2011-01-15'
union
select 4, 2, 6, '2011-01-15'
union
select 5, 2, 4, '2011-01-17'
EDIT3：我做了一些额外的调整来解决其他特殊情况。特别是，之前的代码遇到了以下设置数据的问题，我现在已经更正了：
insert into #orders
select 1, 2, 10, '2011-01-01'
union
select 2, 1, 6, '2011-01-10'
union
select 3, 1, 10, '2011-01-15'
union
select 4, 2, 10, '2011-01-15'
union
select 5, 1, 4, '2011-01-12'