根据第一条消息获取最新的未读消息

Question

我有一个问题从用户之间存储消息的表中检索数据。 下面是表：

CREATE TABLE messages 
(
    id INT UNSIGNED NOT NULL AUTO_INCREMENT, 
    sender VARCHAR(20), 
    recipient VARCHAR(30), 
    content TEXT, 
    is_read TINYINT(1), 
    created_at DATETIME, 
    PRIMARY KEY (`id`) , 
    INDEX idx_sender (sender) , 
    INDEX idx_recipient (recipient) 
); 

INSERT INTO messages (id, sender, recipient, content, is_read, created_at) 
VALUES 
-- Alice and George 
(1, 'Alice', 'George', 'Happy new year', 1, '2012-12-24 23:05:00'), 
(2, 'George', 'Alice', 'It is chrimas night...', 1, '2012-12-24 23:10:00'), 
(3, 'Alice', 'George', 'Happy Xmas then', 0, '2012-12-25 00:00:00'), 
-- John and Paul 
(4, 'John', 'Paul', 'Hi Paul', 1, '2012-12-26 09:00:00'), 
(5, 'Paul', 'John', 'Hi John', 1, '2012-12-26 09:05:00'), 
(6, 'John', 'Paul', 'Have you done this ?', 1, '2012-12-26 09:10:00'), 
(7, 'Paul', 'John', 'No I was unpacking my gifts', 0, '2012-12-26 09:05:00'), 
-- George and Tim 
(8, 'George', 'Tim', 'How was the end of the world ?', 1, '2012-12-22 10:10:00'), 
(9, 'Tim', 'George', 'Really nice !', 0, '2012-12-22 10:15:00'), 
-- John and Tim 
(10, 'John', 'Tim', 'I don\'t know if I should fire you for new year\'s eve', 1, '2012-12-27 15:20:00'), 
(11, 'Tim', 'John', 'That is a great idea!', 0, '2012-12-27 15:20:00'); 

假设乔治和约翰是经理人，和其他人是雇员。 我需要检索管理员是收件人的最新未读邮件。 同时，我必须只检索那些他们也是对话发起人的消息。

那么结果一定是：

• 消息ID 7： '保罗'， '约翰'， '不，我是拆包我的礼物'，0，2012-12-26 9点05分00秒
• message id 9：'Tim'，'George'，'真的很好'，0，2012-12-22 10:15:00
• message id 11：'Tim'，'John'，'That is a '，0，2012-12-27 15:20:00

我做了一个查询，它几乎是，但消息id 11的行缺少：

SELECT 
    m.* 
FROM 
    messages m 
INNER JOIN (
    -- retrieve the first message (fm) 
    SELECT t.sender, t.recipient 
    FROM (
     SELECT id, sender, recipient, content, created_at, 
     IF (sender IN ('John', 'George'), sender, recipient) AS the_other 
     FROM messages 
     WHERE 
      sender IN ('John', 'George') 
      OR recipient IN ('John', 'George') 
     ORDER BY 
     created_at ASC 
    ) t 
    GROUP BY 
     the_other 
    HAVING t.sender IN ('John', 'George') 
) fm ON fm.recipient = m.sender 
    AND fm.sender = m.recipient 
WHERE 
    m.recipient IN ('John', 'George') 
    AND m.is_read = 0 
ORDER BY 
    m.created_at DESC 
LIMIT 0, 10 

由于子查询由the_other分组，因此缺少消息ID 11。 如果我按发件人和收件人分组“发件人”，它将检索邮件标识3，但它是不需要的行。

什么是应该解决我的问题的SQL？

Answer 1

下面可以使用查询：

select a.* from 
(select id, sender, recipient, content, is_read, max(created_at) 
from message where recipent in ('John', 'George') and is_read = 0 
group by sender) a, message b (select id, sender, recipient, 
content, is_read, min(created_at) 
from message where sender in ('John', 'George') group by recipient) b 
where a.sender=b.recepient 

Answer 2

最后，我已经找到了解决办法，但我不认为这是最优化的查询：

SELECT 
    m.* 
FROM 
    messages m 
INNER JOIN (
    -- retrieve the first messages (fm) 

    SELECT t.sender, t.recipient 
    FROM (
     SELECT id, sender, recipient, content, created_at, 
     IF (sender = 'John', recipient, sender) AS the_other 
     FROM messages 
     WHERE 
      sender = 'John' 
      OR recipient = 'John' 
     ORDER BY 
     created_at ASC 
    ) t 
    GROUP BY 
     the_other 
    HAVING t.sender = 'John' 

    UNION 

    SELECT t.sender, t.recipient 
    FROM (
     SELECT id, sender, recipient, content, created_at, 
     IF (sender = 'George', recipient, sender) AS the_other 
     FROM messages 
     WHERE 
      sender = 'George' 
      OR recipient = 'George' 
     ORDER BY 
     created_at ASC 
    ) t 
    GROUP BY 
     the_other 
    HAVING t.sender = 'George' 

) fm ON fm.recipient = m.sender 
    AND fm.sender = m.recipient 
WHERE 
    m.recipient IN ('John', 'George') 
    AND m.is_read = 0 
ORDER BY 
    m.created_at DESC 
LIMIT 0, 10 

http://sqlfiddle.com/#!2/30f5a/17/0