Посчитать процент значения одоной записи от суммы всех записей

select Группа, Количество/(select sum(Количество) from %Tablename% t1 where t1.Группа=t.Группа)
from %Tablename% t

select sum(Количество)/sum(a.Колво)*100 as Procent, Наименов, a.Группа
from MyTable 
inner join
(select sum(Количество) as Колво, Группа
from MyTable 
group by Группа)a
on MyTable.Группа=a.Группа
group by Наименов, a.Группа

select country_id, user_id, ROUND(amount_usd / (select SUM(a.amount_usd)from orders)* 100) as percent
from users
inner join
(select sum(amount_usd) as amount
from orders
group by country_id)a
on users.country_id=a.country_id
group by a.country_id

select
 u.id, u.country_id,
 sum(o.amount_usd) / sum(sum(o.amount_usd)) over (partition by u.country_id)
from
 users u join
 orders o on o.user_id = u.user_id
group by
 u.id, u.country_id;

select
 u.id, u.country_id,
 sum(o.amount_usd * 100) / sum(sum(o.amount_usd)) over (partition by u.country_id) as percent
from
 users u join
 orders o on o.user_id = user_id
group by
 u.id, u.country_id;

SELECT u.id,ROUND((o.amount_usd /SUM(o.amount_usd) OVER (PARTITION BY u.country_id))* 100,2) AS percent
FROM users u 
JOIN
 orders o 
  ON o.user_id = user_id
GROUP BY u.id, u.country_id;

SELECT u.id,ROUND((o.amount_usd /SUM(o.amount_usd) OVER (PARTITION BY u.country_id))* 100,2) AS percent
FROM users u 
JOIN
 orders o 
  ON o.user_id = user_id
GROUP BY u.id, u.country_id;

select
 ROUND((t.amount_usd /SUM(t.amount_usd) OVER (PARTITION BY t.country_id))* 100,2) AS percent
from
 (
  SELECT u.id, u.country_id, o.amount_usd
  FROM users u 
  JOIN
   orders o 
    ON o.user_id = user_id
  GROUP BY u.id, u.country_id
 ) t;