MySQL 行排序依据

Question

我正在尝试从我的 forum_topics 和 forum_replies 表中提取数据以获得最后回复日期；

SELECT 
    (SELECT date FROM forum_replies WHERE topic=5 AND date < NOW()
    ORDER BY id DESC LIMIT 1) as lastreply, 
    ft.* FROM forum_topics ft
    ORDER by lastreply DESC LIMIT 5

它获取记录集中的 lastreply 数据，但不使用它来对记录集中的行进行排序。

我知道为什么，但不确定如何解决？

Answer 1

你应该使用相关子查询：

SELECT 
    (SELECT r.date FROM forum_replies r WHERE ft.topic = r.topic
    ORDER BY r.id DESC LIMIT 1) as lastreply, 
    ft.* 
FROM forum_topics ft
ORDER by lastreply DESC LIMIT 5

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