SQLite：如何優化UNION查詢

Question

在下面的UNION查詢中，子查詢＃2和＃4是相同的，我想避免兩次提取相同的數據。據我所知，有些數據庫允許你在查詢開頭寫一個WITH子句，但SQLite不支持它。所以問題是如何優化下面的查詢。

SELECT disliked, recommended, weight 
FROM 
    (SELECT movie_id * 10 AS disliked 
     FROM ratings 
    WHERE rating IN (1,2) 
     AND user_id = #{id} 
    ) AS input, 
    (SELECT movie_id AS recommended, rating AS weight 
     FROM ratings 
    WHERE rating IN (4,5) 
     AND user_id = #{id} 
    ) AS output 
UNION 
SELECT liked, recommended, weight 
FROM 
    (SELECT movie_id, movie_id * 10 + 1 AS liked 
     FROM ratings 
    WHERE rating IN (4,5) 
     AND user_id = #{id} 
    ) AS input, 
    (SELECT movie_id AS recommended, rating AS weight 
     FROM ratings 
    WHERE rating IN (4,5) 
     AND user_id = #{id} 
    ) AS output 
WHERE input.movie_id != recommended 

Answer 1

我不能完全肯定我讀過你的權利這快，但也許是這樣的：

SELECT 
    10*r1.movie_id+(r1.rating IN (4,5)) AS likedisliked, 
    r2.movie_id AS recommended, 
    r2.rating AS weight 
FROM ratings AS r1, ratings AS r2 
WHERE r1.user_id=#{id} AND r2.user_id=#{id} 
     AND r1.rating IN (1,2,4,5) AND r2.rating IN (4,5) 
     AND r1.movie_id!=r2.movie_id;