SQL Server 2019

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CREATE TABLE MyTable (
  Category VARCHAR(10),
  Qty INT,
  Orders INT,
  Level INT
);

INSERT INTO MyTable
  (Category, Qty, Orders, Level)
VALUES
  ('Product',null,null, 0),
  ('Part 1', 2, 4, 1),
  ('Part 2', 3,5,1),
  ('Part 1.1', 4,6,2),
  ('Part 1.2', 5,7,2);
  
select * from MyTable;  


Category
Qty
Orders
Level


Product
null
null
0

Part 1
2
4
1

Part 2
3
5
1

Part 1.1
4
6
2

Part 1.2
5
7
2


Update myTable
   set Qty = agg.Qty, Orders=agg.Orders 
from (Select Sum(Qty) Qty, Sum(Orders) Orders from myTable where level > 0) agg
where level=0;
1 rows affected

select * from myTable;


Category
Qty
Orders
Level


Product
14
22
0

Part 1
2
4
1

Part 2
3
5
1

Part 1.1
4
6
2

Part 1.2
5
7
2

Category	Qty	Orders	Level
Product	null	null	0
Part 1	2	4	1
Part 2	3	5	1
Part 1.1	4	6	2
Part 1.2	5	7	2