Creating sequential date ranges for items in a queue - sql-server

Question

I have a table 'item_queue' containing, items, groups and a sequence number.
Each item is unique and is held against a group with a number indicating the sequence. The count is a total for that item e.g.
group_id|item_id|sequence_order_number|count
--------------------------------------------
A |123 |1 |20
A |124 |2 |30
B |125 |1 |10
Given this information I am trying to set up sequential start and end dates
The start datetime of the first item for a group is the current time, for example assume start of item 123 is '2019-04-04 12:00:00.000' then
end datetime would be start + (count * minutes) so '2019-04-04 12:20:00.000'
The start of item 124 would equal that end date as it is the next in the sequence for that group. the end is then calculated the same way to be '2019-04-04 12:50:00.000'
item 125 would start the time again at '2019-04-04 12:00:00.000' as it is in a different group
I have attempted a few ways to do this, and I think the answer is a recursive cte, but I can't wrap my head around it to make it work for one or multiple groups, my unsuccessful attempt for a single group:
;with cte as
(
select
group_id,
item_id,
count,
GETDATE() as start_datetime,
DATEADD(MINUTE, count, GETDATE()) as end_datetime,
iq.sequence_order_number
from item_queue iq
where iq.group_id = 'A'
union all
select
group_id,
item_id,
count,
cte.end_datetime,
DATEADD(MINUTE, count, cte2.end_datetime) as end_datetime,
iq.sequence_order_number
from item_queue iq
inner join cte
on cte.group_id = iq.group_id
and cte.sequence_order_number > iq.sequence_order_number
where iq.group_id = 'A'
)
select * from cte
I suspect the answer may involve a row number window something like
ROW_NUMBER() OVER (Partition By iq.group_id Order By iq.sequence_order_number ASC)
But I have had trouble using it recursively.
Using SQL server 2012, without the ability to upgrade this database.

Answer 1

The minutes you want to add are practically a cumulative sum. The sum() over() window function is available in 2012 and performs exactly that. Try:
select
*,
isnull(sum([count]) over
(
partition by group_id
order by item_id asc
rows between unbounded PRECEDING and 1 PRECEDING
)
,0) as cum_count_start,
sum([count]) over ( partition by group_id order by item_id asc ) as cum_count_end
from item_queue
You already know how to use dateadd after this point.
What the individual window function caluses do:
partition by group_id : Seperate (partition) the calculations for each group_id value subset
order by item_id asc : make a virtual sorting of the rows on which the window range will be applied
rows between.... : The actual window. For the start date, we want to consider all the lines from the start (thus unbounded preceding) to the previous one (thus 1 preceding), since you don't want the start date to include the current line's [count]. Note that ommitting this clause like we did on the cum_count_end is equivelant to rows between unbounded preceding and current row.
The isnull(...,0) is needed because for the first line of each group_id you want to add 0 to the start date, but the window function sees no rows and returns NULL, so we need to change this to 0.