Question

我正在创建一个小型消息传递平台，我有三个表：

`msg_conversations`
+-------+---------------------+------+-----+---------+----------------+
| Field | Type                | Null | Key | Default | Extra          |
+-------+---------------------+------+-----+---------+----------------+
| id    | int(10) unsigned    | NO   | PRI | NULL    | auto_increment |
+-------+---------------------+------+-----+---------+----------------+

`msg_participants`
+------------+---------------------+------+-----+---------+----------------+
| Field      | Type                | Null | Key | Default | Extra          |
+------------+---------------------+------+-----+---------+----------------+
| id         | int(10) unsigned    | NO   | PRI | NULL    | auto_increment |
| convo_id   | int(10) unsigned    | NO   | MUL | NULL    |                |
| prof_id    | int(10) unsigned    | NO   | MUL | NULL    |                |
| prof_type  | tinyint(3) unsigned | NO   |     | NULL    |                |
| is_visible | tinyint(3) unsigned | NO   |     | NULL    |                |
+------------+---------------------+------+-----+---------+----------------+

`msg_messages`
+----------------+---------------------+------+-----+---------+----------------+
| Field          | Type                | Null | Key | Default | Extra          |
+----------------+---------------------+------+-----+---------+----------------+
| id             | int(10) unsigned    | NO   | PRI | NULL    | auto_increment |
| participant_id | int(10) unsigned    | NO   | MUL | NULL    |                |
| message        | text                | NO   |     | NULL    |                |
| time_cre       | int(11)             | NO   |     | NULL    |                |
+----------------+---------------------+------+-----+---------+----------------+

我想返回最新消息（通过msg_messages.time_cre）以及参与者所在的每个对话的convo_id。

我的逻辑如下：1）获取用户正在参与的所有会话ID（由msg_participants.prof_type和msg_participants.prof_id定义）。 2）根据每个会话ID获取每个参与者。 3）在每个对话中返回最后一条消息（按msg_messages.time_cre排序）。该功能应类似于Facebook返回会话列表以及每个会话中最后一条消息的预览。

到目前为止，这是我的代码：

SELECT msg_participants.convo_id,
    msg_messages.id,
    msg_participants.prof_id,
    msg_participants.prof_type,
    SUBSTRING(msg_messages.message,1,50) as message,
    msg_messages.time_cre
FROM msg_participants,msg_messages
WHERE
    msg_participants.id IN 
        (SELECT participant_id FROM 
            (SELECT * FROM (SELECT msg_messages.participant_id,msg_participants.convo_id FROM msg_messages,msg_participants WHERE msg_messages.participant_id=msg_participants.id AND participant_id IN 
                (SELECT id FROM msg_participants WHERE convo_id IN (SELECT convo_id FROM msg_participants WHERE prof_id=PROFILE_ID_HERE AND prof_type=PROFILE_TYPE_HERE))
            ORDER BY time_cre DESC) AS tbl1
            GROUP BY tbl1.convo_id) AS tbl2)

    AND msg_messages.id IN 
        (SELECT id FROM (SELECT * FROM (SELECT msg_messages.id,msg_participants.convo_id FROM msg_messages,msg_participants WHERE msg_messages.participant_id=msg_participants.id AND participant_id IN 
                (SELECT id FROM msg_participants WHERE convo_id IN (SELECT convo_id FROM msg_participants WHERE prof_id=PROFILE_ID_HERE AND prof_type=PROFILE_TYPE_HERE)) 
            ORDER BY time_cre DESC) AS tbl3
            GROUP BY tbl3.convo_id) AS tbl4)

我知道我的代码是多么丑陋/ hacky，但是我已经多次重写它了，我只想要一些有效的东西。这段代码是......关闭。但它返回msg_participants.id X msg_messages.id的笛卡尔积。对我来说，为什么它会返回笛卡尔积，这是有意义的，但我不确定如何使这个查询做我想做的事。

谢谢！

Answer 1

您可以使用not exists从msg_messages为给定participant_id

选择最近一行

select mp.convo_id, mm.id, mp.prof_id, mp.prof_type, mm.message, mm.time_cre
from msg_participants mp
join msg_messages mm on mp.id = mm.participant_id
where mp.prof_id = PROF_ID_HERE
and mp.prof_type = PROF_TYPE_HERE 
and not exists (
    select 1 from msg_messages mm2
    where mm2.participant_id = mm.participant_id
    and mm2.time_cre > mm.time_cre
)

MySQL Query采用笛卡尔积

1 个答案: