How to SUM() over column with reset condition?

Question

I\'m using Postgresql 9.2 and I need to sum quantitys from buttom up with initial value of 100, however if I encounter a row with name X i need to restart the SUM from the

Answer 1

To reset a Windowed Aggregate based on a condition you need to add another level to define the different groups of rows:

with cte as
 (
   select itemorder, name, qty,
      case -- start qty for each group
         when itemorder = max(itemorder) over () then 100 -- newest row
         when name = 'X' then qty
         else 0
      end 
           -- only sum 'A' rows
      + case when name = 'A' then qty else 0 end as new_qty, 

           -- grp value increases only when name = 'X'
           -- this assigns the same value to each row in a group
           -- will be used in the next step to PARTITION BY
      sum(case when name = 'X' then 1 else 0 end) 
      over (order by itemorder desc
            rows unbounded preceding) as grp
   from myTable
 )

select itemorder, name, qty,
   sum(new_qty)
   over (partition by grp
         order by itemorder desc
         rows unbounded preceding ) as modified_sum
from cte
order by item order

See fiddle