Use expression in window function that references columns from the current row Suppose I have the following query that uses window function:

SELECT id
     , var
     , num
     , SUM(var * num) OVER (ORDER BY id ROWS BETWEEN 2 PRECEDING AND CURRENT ROW) AS calc
FROM (VALUES
    (1, 0.1, 7),
    (2, 0.7, 1),
    (3, 0.3, 9),
    (4, 0.9, 5),
    (5, 0.5, 3)
) AS t(id, var, num)


And the following result:

id | var | num | calc | explanation
1  | 0.1 | 7   | 0.7  | 0.1*7
2  | 0.7 | 1   | 1.4  | 0.1*7 + 0.7*1
3  | 0.3 | 9   | 4.1  | 0.1*7 + 0.7*1 + 0.3*9
4  | 0.9 | 5   | 7.9  | 0.7*1 + 0.3*9 + 0.9*5
5  | 0.5 | 3   | 8.7  | 0.3*9 + 0.9*5 + 0.5*3


Is is possible to reference the var column from the outside inside the SUM() OVER ()? For example:

id | var | num | calc | sum of f(r.var, w.var, w.num)
1  | 0.1 | 7   | ...  | iif(0.1&lt;=0.1,0.1,0.1)*7
2  | 0.7 | 1   | ...  | iif(0.7&lt;=0.1,0.7,0.1)*7 + iif(0.7&lt;=0.7,0.7,0.7)*1
3  | 0.3 | 9   | ...  | iif(0.3&lt;=0.1,0.3,0.1)*7 + iif(0.3&lt;=0.7,0.3,0.7)*1 + iif(0.3&lt;=0.3,0.3,0.3)*9
4  | 0.9 | 5   | ...  | iif(0.9&lt;=0.7,0.9,0.7)*1 + iif(0.9&lt;=0.3,0.9,0.3)*9 + iif(0.9&lt;=0.9,0.9,0.9)*5
5  | 0.5 | 3   | ...  | iif(0.5&lt;=0.3,0.5,0.3)*9 + iif(0.5&lt;=0.9,0.5,0.9)*5 + iif(0.5&lt;=0.5,0.5,0.5)*3


Basically I want to calculate running sum of an expression that references var column in the current row and var column in the current window. iif and multiplication is just an example.

The solution should work in SQL Server but generic answer would be best.

Here is a db&lt;>fiddle where I was able to achieve the result with correlated queries but I want to use window functions.
Is this what you are looking for ?

create table #t(id int, vars decimal(3,1), num int)
insert into #t VALUES
    (1, 0.1, 7),
    (2, 0.7, 1),
    (3, 0.3, 9),
    (4, 0.9, 5),
    (5, 0.5, 3)


;With CTE as
(
select
      t.id
     ,t.vars
     ,t.num
     ,isnull(lag(vars ,1) OVER (ORDER BY id ),0) AS vars1
     ,isnull(lag(vars ,2) OVER (ORDER BY id ),0) AS vars2
     ,isnull(lag(num ,1) OVER (ORDER BY id ),0) AS num1
     ,isnull(lag(num ,2) OVER (ORDER BY id ),0) AS num2
FROM  #t t
)

select * 
,iif(vars&lt;=0.1,vars,vars2)*num2 + 
iif(vars&lt;=vars1,vars,vars1)*num1 + 
iif(vars&lt;=vars,vars,vars)*num
from CTE

drop table #t