行数没有区别

Question

我有这个问题：

SELECT DISTINCT ON (pair) *,
    CASE WHEN sender_id<recipient_id
      THEN (sender_id,recipient_id)
      ELSE (recipient_id,sender_id)
    END AS pair FROM "messages" WHERE 
    ((recipient_id = 6 and recipient_delete = false) or 
    (sender_id = 6 and sender_delete = false))  
    GROUP BY messages.id
    ORDER BY  pair, created_at, unread_count DESC

带来用户对话的最新消息（用户收件箱）。

现在我需要显示每个对话的未读消息总数，“已打开”表中有一个标志，表示消息是否未读。

这是我为它写的查询：

SELECT DISTINCT ON (pair) *,
    CASE WHEN sender_id<recipient_id
      THEN (sender_id,recipient_id)
      ELSE (recipient_id,sender_id)
    END AS pair, COUNT(messages.opened= false) AS unread_count FROM "messages" WHERE 
    ((recipient_id = 6 and recipient_delete = false) or 
    (sender_id = 6 and sender_delete = false))  
    GROUP BY messages.id
    ORDER BY  pair, created_at, unread_count DESC

但即使有超过 2 或 3 条消息，它也会显示计数为 1（我猜这是因为 Distinct）。

有什么想法吗？

如果您想知道为什么我们有用户CASE WHEN 功能，请查看此question：

Answer 1

我有点困惑，因为您的逻辑似乎会带来 最早 消息而不是最新消息。

虽然distinct on 很强大，但我不确定你是否能轻易得到你想要的。我的倾向是改用窗口函数：

SELECT m.*
FROM (SELECT m.*,
             (CASE WHEN sender_id < recipient_id
                   THEN (sender_id, recipient_id)
                   ELSE (recipient_id, sender_id)
              END) AS pair,
             ROW_NUMBER() OVER (PARTITION BY (CASE WHEN sender_id < recipient_id
                                                   THEN (sender_id, recipient_id)
                                                   ELSE (recipient_id, sender_id)
                                              END)
                                ORDER BY created_at, unread_count DESC
                               ) as seqnum,
             SUM(CASE WHEN m.opened = false THEN 0 ELSE 1 END) OVER
                 (PARTITION BY (CASE WHEN sender_id < recipient_id
                                     THEN (sender_id, recipient_id)
                                     ELSE (recipient_id, sender_id)
                                END)
                 ) as NumUnopened
      FROM "messages" m
      WHERE ((recipient_id = 6 and recipient_delete = false) or 
             (sender_id = 6 and sender_delete = false))  
     ) t
WHERE seqnum = 1;

Answer 2

在您的查询中选择字段“messages.opened”（别名messages_opened），然后像这样包围它们：

选择计数（messages_opened） FROM (... [your query] ) as A 其中 A.messages_opened = false

查询示例：

将@table 声明为表 ( messageid INT， 发件人 INT， 收件人 INT ) 插入@table （消息ID， 发件人ID， 收件人ID） 价值观（1， 10, 20), (2, 10, 20), (3, 11, 20), (4, 11, 20), (5, 10, 20) 选择 Count(messageid) 作为 countOfMessages， 发件人ID， 收件人ID 来自@表 按发件人ID分组， 收件人ID