sum(сумма)over(partition by ученик)

USE master
GO

WITH STUDENTS(ID,NAME,CREDIT) AS
(
  SELECT 1,'IGOR',100
   UNION ALL
  SELECT 2,'IGOR',500 
   UNION ALL
  SELECT 3,'IGOR',132
   UNION ALL
  SELECT 4,'IVAN',420
   UNION ALL
  SELECT 5,'IVAN',380
   UNION ALL
  SELECT 6,'VASYA',110
   UNION ALL
  SELECT 7, 'VASYA',98     
),
GROUPEDSTUDENTS AS
(
  SELECT S.NAME,SUM(S.CREDIT)TOTALCREDIT
   FROM STUDENTS S
  GROUP BY S.NAME 
)
SELECT S.ID,S.NAME,S.CREDIT,CAST(S.CREDIT AS VARCHAR(3))+'/'+CAST(GR.TOTALCREDIT AS VARCHAR(5))TT
FROM STUDENTS S
JOIN GROUPEDSTUDENTS GR ON S.NAME=GR.NAME

create table #T 
( 
id int, 
col varchar(20), 
Num int 
) 

insert into #T (id,col,Num)  
values (1,'Вася',100),(2,'Вася',250),(3,'Вася',120),(4,'Петя',400),(5,'Вася',260),(6,'Петя',280) 

select id,col,num, cast(100. * Num/ sum(num) over(partition by col) as numeric (5,2)) as Procent, 
from #T 

drop table #T