mysql - 使用 JOINS 代替子查询

Question

我有一个使用子查询的简单查询：

SELECT pictures.*
FROM pictures
WHERE pictures.user_id IN
    (SELECT follows.following_id
     FROM follows
     WHERE follows.follower_id = 9)
ORDER BY created_at DESC LIMIT 5;

我想知道，a）如何删除子查询并改用 JOINS，b）使用 JOINS 代替子查询会带来性能优势吗？

（follows.following_id、follows.follower_id、pictures.user_id 都被索引）

谢谢

Answer 1

SELECT  DISTINCT pictures.*
FROM    pictures
        INNER JOIN follows
            ON pictures.user_ID = follows.following_id
WHERE   follows.follower_id = 9
ORDER   BY pictures.created_at DESC 
LIMIT   5

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• SQL 连接的可视化表示

更新

实现相同结果的另一种方法是使用EXISTS

SELECT  *
FROM    pictures
WHERE   EXISTS
        (
            SELECT  1
            FROM    follows
            WHERE   pictures.user_ID = follows.following_id AND
                    follows.follower_id = 9
        )
ORDER   BY pictures.created_at DESC 
LIMIT   5