select
    root_message.id,root_message.active,root_message.subject,case
        when max_reply_id.max_id is null then 
            root_message.message
        else
            reply_message.message
    end as message,root_message.datetime,root_message.reply,root_message.from,root_message.to,root_message.viewed,root_message.archived
from
    -- basic data
    cms_messages as root_message
    -- ID of last reply for every root message
    left join (
        select 
            max(id) as max_id,reply as parent_id 
        from 
            cms_messages
        where
            reply <> 0 
        group by 
            reply
    ) as max_reply_id on max_reply_id.parent_id = root_message.id                                              
    left join cms_messages as reply_message on reply_message.id = max_reply_id.max_id
where
    root_message.reply = 0

它使用子查询max_reply_id作为数据源来选择最新答案的ID.如果存在(即,如果有答案),则使用reply_message.message.如果不存在(根消息未找到答案),则使用root_message.message.

你也应该考虑表的结构.例如,如果回应包含NULL(如果是父消息)或现有消息的ID,则会更有意义.目前,您将其设置为0(不存在的消息的ID),这是错误的.查看和归档的类型也很奇怪.

编辑：你也应该避免使用having子句.在可能的地方使用.

这是一个满足您的要求的新查询.如果它有任何问题(即如果它返回错误的数据),让我知道.

像第一个查询一样,它：

>使用子查询reply_summary来累积关于回复的数据(最后回复的ID,回复的数量和未读回复的数量);
>将此子查询加入到基表中;
>根据reply_summary.max_reply_id将cms_messages作为reply_message连接到子查询,以获取有关最后一个回复(消息,datetime)的数据.

我已经简化了你如何确定last_datetime的方式 – 它现在需要上次回复的时间(如果有任何回复)或原始帖子的时间(当没有发现回复时).

我没有过滤回复和来自字段.如果有必要,应该更新reply_summary子查询的where子句.

select
    parent_message.id,parent_message.subject,parent_message.message,parent_message.from,parent_message.to,coalesce(reply_summary.num_replies,0) as num_replies,last_reply_message.datetime as reply_datetime,(parent_message.archived NOT LIKE '%,{$cms_user['id']},(parent_message.viewed   LIKE     '%,reply_summary.unread_replies,coalesce(last_reply_message.message,parent_message.message) as last_message,coalesce(last_reply_message.datetime,parent_message.datetime) as last_datetime
from
    cms_messages as parent_message
    left join (
        select
            reply as parent_id,max(id) as last_reply_id,count(*) as num_replies,sum(viewed not like '%,%') as unread_replies
        from
            cms_messages
        where
            reply <> 0 and
            active = 1
        group by
            reply
    ) as reply_summary on reply_summary.parent_id = parent_message.id
    left join cms_messages as last_reply_message on last_reply_message.id = reply_summary.last_reply_id
where
    parent_message.reply = 0 and
    parent_message.active = 1 and
    (parent_message.to like '%,%' or parent_message.to = 'all' or parent_message.from = '{$cms_user['id']}')
order by
    last_datetime desc;