WITH t1 AS
(SELECT User, Rating, Count(*) AS n
FROM your_table
GROUP BY User, Rating)
SELECT User, Rating, n,
(0.0+n)/(COUNT(*) OVER (PARTITION BY User)) -- no integer divide!
FROM t1;
Nebo
SELECT User, Rating, Count(*) OVER w_user_rating AS n,
(0.0+Count(*) OVER w_user_rating)/(Count(*) OVER (PARTITION BY User)) AS pct
FROM your_table
WINDOW w_user_rating AS (PARTITION BY User, Rating);
Chtěl bych zjistit, zda jeden z těchto nebo druhých poskytuje lepší plán dotazů s vhodným nástrojem pro vaše RDBMS.