codice:
DECLARE @tbl TABLE
(
  Qta          float
, Cod_Art      varchar(20)
, DDif         varchar(20)
);


INSERT INTO @tbl ( Cod_Art, Qta,  DDif )
VALUES
('A',   10,  '01/01/2020'),
('A',   8 ,  '02/01/2020'),
('A',   12,  '03/01/2020'),
('B',   20,  '01/01/2020'),
('B',   18,  '02/01/2020'),
('B',   16,  '03/01/2020'),
('C',   30,  '01/01/2020'),
('C',   40,  '02/01/2020'),
('C',   50,  '03/01/2020')
;




SELECT  
DDif, 
SUM(CASE WHEN Cod_Art = 'A' THEN Qta ELSE 0 END) AS Ax,
SUM(CASE WHEN Cod_Art = 'B' THEN Qta ELSE 0 END) AS Bx,
SUM(CASE WHEN Cod_Art = 'C' THEN Qta ELSE 0 END) AS Cx,
((SUM(CASE WHEN Cod_Art = 'A' THEN Qta ELSE 0 END)) + (SUM(CASE WHEN Cod_Art = 'B' THEN Qta ELSE 0 END))) / (SUM(CASE WHEN Cod_Art = 'C' THEN Qta ELSE 0 END)) AS Raoporto
FROM @tbl 
GROUP BY 
DDif
;