Select quantity on a 1st table based on a total quantity the 2nd table - sql

Question

Table 1
ID
Grp
Qty
1
A
5
2
A
4
3
B
5
4
B
3
5
B
2
6
C
14
7
D
1
8
D
1
9
E
2
10
E
2
11
E
1
12
E
1
Table 2
ID
Grp
Qty
1
A
7
2
B
9
3
C
13
4
D
1
5
E
4
Select/Output
ID
Grp
Qty
1
A
0
2
A
2
3
B
0
4
B
0
5
B
1
6
C
1
7
D
0
8
D
1
9
E
0
10
E
0
11
E
1
12
E
1
I want to select a row on a 1st table with a specific quantity based on the total quantity of the 2nd table. The result is on the 3rd table. Please see sample tables above, I really appreciate a help, thank you so much and sorry it was my first time asking a question here.
I have tried this code on both 2 tables
WITH tbl AS(
SELECT ID,
Qty,
Grp,
ROW_NUMBER() OVER (PARTITION BY Grp)AS Rown,
SUM(Qty) OVER (PARTITION BY Grp)AS Total
FROM Table1
)
SELECT * FROM tbl WHERE Rown = 1
But I am not able to select the specific rows on Table 1 because it only select the 1st row and total the quantity. Every row on table 1 has its own quantity.

Answer 1

You could use a cumulative windowed aggregates and then a CASE expression to achieve this:
--Saple Data
WITH Table1 AS(
SELECT *
FROM (VALUES(1,'A',5),
(2,'A',4),
(3,'B',5),
(4,'B',3),
(5,'B',2),
(6,'C',14))V(ID,Grp,Qty)),
Table2 AS(
SELECT *
FROM (VALUES(1,'A',7),
(2,'B',9),
(3,'C',13))V(ID,Grp,Qty)),
--Solution
CTE AS(
SELECT T1.ID,
T1.Grp,
T1.Qty,
SUM(T1.Qty) OVER (PARTITION BY T1.Grp ORDER BY T1.Id
ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS RunningQty,
T2.Qty AS T2Qty
FROM Table1 T1
JOIN Table2 T2 ON T1.Grp = T2.Grp)
SELECT C.ID,
C.Grp,
CASE WHEN C.RunningQty <= C.T2Qty THEN C.Qty
ELSE C.T2Qty - LAG(C.RunningQty,1,0) OVER (PARTITION BY C.Grp ORDER BY C.ID)
END AS Qty
FROM CTE C;