Question

我在从用户之间存储消息的表中检索数据时遇到问题。这是表格：

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George和John是经理，其他人是员工。我需要检索管理者是收件人的最新未读邮件。与此同时，我必须只检索那些也是对话发起者的消息。

所以结果必须是：

消息ID 7：'保罗'，'约翰'，'不，我正在打开礼物'，0,2012-12-26 09:05:00
消息ID 9：'蒂姆'，'乔治'，'真的很好'，0,2012-12-22 10:15:00
消息ID 11：'蒂姆'，'约翰'，'这是一个好主意！'，0,2012-12-27 15:20:00

我做了一个几乎是查询的查询，但是消息ID 11的行丢失了：

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

缺少消息ID 11，因为子查询是由the_other分组的。如果我将发件人和收件人分开“分组依据”，它将检索邮件ID 3，但这是一个不需要的行。

什么应该是解决我问题的SQL？

Answer 1

可以使用以下查询：

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

最后我找到了一个解决方案，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

根据第一条消息获取最新的未读消息

2 个答案: