GROUPBYクエリを使用したパーセンテージの計算

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;

または

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);

これらのいずれかが、RDBMSに適したツールを使用してより優れたクエリプランを生成するかどうかを確認します。