SQLite：如何优化 UNION 查询

Posted 2023-04-14

【中文标题】SQLite：如何优化 UNION 查询

【英文标题】：SQLite: How to optimize a UNION query

【发布时间】：2012-09-26 20:50:14

【问题描述】：

在下面的 UNION 查询中，子查询 #2 和 #4 是相同的，我想避免两次获取相同的数据。据我所知，有些数据库允许你在查询开头写一个WITH 子句，但SQLite 不支持。所以问题是如何优化下面的查询。

 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

【参考方案1】：

我不完全确定我这么快就读对了你，但可能是这样的：

 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;