需要有关MySQL SELECT的帮助（组内聚合）

Question

背景

我有一张评论表。每条评论都属于'lead'表或'prod'表中的特定行。 “lead”和“prod”表中的每一行都可以有多个注释。每行也可以有多个相同的'kstatus'注释。

comments
    INT id /* primary key */
    ENUM('lead', 'prod') type /* tells me whether the comment belongs to a row in the lead table or the prod table */
    INT fid /* foreign key, refers to the id column in either the lead or prod table */
    MEDIUMTEXT comment  /* the comment itself */
    INT timestamp /* when the comment was written, i.e. 1300530201 */
    INT kstatus /* foreign key, refers to the id column in the kstatus column */

lead
    INT id
    ...

prod
    INT id
    ...

kstatus
    INT id
    ...

type ='lead'和fid = 105的注释引用lead的行，其中lead.id = 105。 type ='prod'和fid = 105的注释引用prod.id = 105的prod行。

问题

我想选择带有特定kstatus（比如14）的'lead'表中每一行给出的第一条评论（如果有的话，并根据时间戳）。

这是我的尝试，返回的结果太少了。也许是由于fid的混合引用了lead.id和fid指的是prod.id？

SELECT C1.*
FROM comments AS C1
LEFT JOIN comments AS C2 ON (C1.timestamp > C2.timestamp AND C1.fid = C2.fid)
WHERE C2.timestamp is NULL
AND C1.type = 'lead'
AND C1.kstatus = 14
GROUP BY C1.fid

Answer 1

select c2.*
from (
    select min(c.id) realfirstcomment
    from (
        select fid, min(timestamp) as firstcomment
        from comments
        where type='lead' and kstatus=14
        group by fid
    ) f, comments c
    where f.fid = c.fid
      and c.type='lead' and c.kstatus=14
      and f.firstcomment = c.timestamp
    group by c.fid
) f2, c2
where f2.realfirstcomment = c2.id

Answer 2

由于产品和潜在客户之间的混合，您不仅会失去结果，而且在kstatus值之间也是如此 - 您的左连接会在C2中查找具有较早时间戳的所有行，无论类型和kstatus如何因为fid是相同的。

一种解决方案是将AND C1.type = C2.type AND C1.kstatus = C2.kstatus添加到连接条件。