CALCOLARE LA PERCENTUALE IN SQL per gruppo

Utilizzando OVER(PARTITION BY ), non avrai bisogno di un join in SQL Server

SELECT *,  100*Grade/ SUM(Grade) OVER(PARTITION BY ID) AS Percentage
FROM (
     VALUES(1, 10),(1,20), (2, 15), (2,24), (2,16), (3,29)
) your_table (ID, GRADE)