mysql - 优化从3个表mysql中选择元素的查询

Question

数据库如下:

Classes       Challenges         Class Challenges
id             id                  id
               title               class_id
                                   challenge_id

为了获得特定类(class)的所有挑战，我使用以下内容

SELECT 
    DISTINCT class_challenges.challenge_id, 
    challenges.title
FROM class_challenges 
LEFT JOIN challenges 
    ON class_challenges.challenge_id = challenges.id
WHERE class_challenges.class_id = :class_id
ORDER BY challenge_id

如何对不属于特定类别的所有挑战执行相同的操作？ 到目前为止我使用:

SELECT 
    DISTINCT challenges.id,
    challenges.title 
FROM 
    challenges,
    class_challenges
WHERE challenges.id NOT IN(
                            SELECT 
                                DISTINCT class_challenges.challenge_id
                            FROM class_challenges
                            LEFT JOIN challenges 
                                ON class_challenges.challenge_id = challenges.id
                            WHERE class_challenges.class_id = :class_id
                            ORDER BY challenge_id
                        );

我认为可以写得更好。 (也许使用双连接？)

那么，如何优化(如果可以的话？)

Answer 1

尝试这个查询

SELECT
    t.id,
    t.title,
    t.CCID
FROM
(
    SELECT      
        challenges.id,
        challenges.title,
        class_challenges.id as CCID                 
    FROM 
        challenges
    LEFT JOIN class_challenges 
        ON class_challenges.challenge_id = challenges.id
) as t  
WHERE t.CCID IS NULL