Sum function in CTE recursive in sql

Question

Month    | Value1  | data    | Excel formula
---------+---------+---------+--------------
March-19 | 30 (B2) | 30 (C2) | = (B2)-sum(0) 
April-19 | 30 (B3) |  0 (C3) | = (B3)-sum(C2) 
May-19   | 30 (B4) |  0 (C4) | = (B4)-sum(C2:C3)
June-19  | 30 (B5) |  0 (C5) | = (B5)-sum(C2:C4)

We need to calculate the c column that is DATA column. Formula for calculating the data column is present in Excel formula. Any cell value of data column == (is value of a fixed column- sum of all the previous values of that calculated column).

Basically we need to use CTE recursive with the sum function.

How can we do that?

Answer 1

I don't think that you need recursion here, window functions should do it:

select
    t.*,
    value1 - sum(data) over(
        order by month
        rows between unbounded preceding and 1 preceding
    ) res
from mytable t

This assumes that you are storing dates in the month column in a date -like datatype (or at least in a string format that is properly sortable).

We can also express the query as follows, without a range clause:

select
    t.*,
    value1 + data - sum(data) over(order by month) res
from mytable t