mysql - SQL查询优化，如果第一个为空则按其他行排序

Question

SQL 查询:

SELECT 
    T.*,
    U.nick AS author_nick,
    P.id AS post_id,
    P.name AS post_name,
    P.author AS post_author_id,
    U2.nick AS post_author
FROM 
    zero_topics T
LEFT JOIN 
    zero_posts P
ON 
    T.id = P.topic_id
LEFT JOIN 
    zero_players U
ON 
    T.author = U.uuid
LEFT JOIN 
    zero_players U2
ON 
    P.author = U2.uuid
ORDER BY 
    P.id DESC

问题:

1. 我需要双重左连接才能从主题和帖子的 UUID 中获取用户昵称
2. 并非所有主题都会有帖子，如您所见，我根据帖子 ID 排序(它将是日期)，但它显示在最后一个帖子的第一名主题上，以及没有回复的底部主题上，当帖子没有时，我如何定义顺序不存在？

Answer 1

1.如果需要在不同列中显示昵称，则需要双左连接

2.您可以在order by中使用case

ORDER BY 
    CASE 
        WHEN P.id is null THEN T.ID
        ELSE P.ID 
    END ASC

最终查询:-

SELECT 
    T.*,
    U.nick AS author_nick,
    P.id AS post_id,
    P.name AS post_name,
    P.author AS post_author_id,
    U2.nick AS post_author
FROM 
    zero_topics T
LEFT JOIN 
    zero_posts P
ON 
    T.id = P.topic_id
LEFT JOIN 
    zero_players U
ON 
    T.author = U.uuid
LEFT JOIN 
    zero_players U2
ON 
    P.author = U2.uuid
ORDER BY 
CASE 
    WHEN P.id is null THEN T.ID
    ELSE P.ID 
END ASC