I think you would need to use a recursive common table expression.
EDIT: I've also added a procedural implementation further below instead of using a recursive common table expression. I recommend using that procedural approach, as I think there may be a number of data scenarios that the recursive CTE query that I've included probably doesn't handle.
The query below gives the correct answers for the scenarios that you've provided, but you would probably want to think up some additional complex scenarios and see whether there are any bugs.
For instance, I have a feeling that this query may break down if you have multiple previous orders overlapping with a later order.
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 mentioned a concern about the recursion limit on common table expressions. Below is a procedural alternative using a table variable and a while loop, which I believe accomplishes the same thing.
While I've verified that this alternative code does work, at least for the sample data provided, I'd be glad to hear anyone's comments on this approach. Good idea? Bad idea? Any concerns to be aware of?
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: I've made some adjustments to the code above to address various special cases, such as if there just happen to be two orders for a user that both end on the same date.
For instance, changing the setup data to the following caused issues with the original code, which I've now corrected:
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: I've made some additional adjustments to address other special cases. In particular, the previous code ran into issues with the following setup data, which I've now corrected:
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'
