提琴

Question

我担心我错误地考虑了SQL。我可以将查询混合在一起做我想做的事，但是它们最终由于嵌套了多个选择而变得非常荒谬。这是一个我无法理解如何简化的示例（更不用说如何提高性能了）。我的主要问题之一是当我将用户分组在一起时如何从工会中挑选最新消息。将其包装在另一个选择中只是为了使其正常工作，但看起来太难看了，我一直在担心这样做的性能。我对查询功能的简单英语解释是：

查看与给定用户相关的已发送和已接收消息（在本例中为硬编码为用户ID 1），并返回发送消息给我的所有用户的唯一列表，以及最近发送消息或发送消息的时间。我们之间收到的。

不幸的是，我存储在另一个元表中的uid字段也必须返回，这使整个事情变得更加复杂：

SELECT ID, user_nicename, MAX(last_message) AS last_message, uid FROM
    (SELECT * FROM 
        (SELECT wp_users.ID, wp_users.user_nicename, MAX(time) as last_message, (SELECT meta_value FROM wp_usermeta WHERE user_id = recipient_user_id AND meta_key = 'uid') AS uid FROM `wp_usermessages`
            LEFT JOIN wp_users ON wp_users.ID = recipient_user_id
            WHERE sender_user_id = 1
            GROUP BY recipient_user_id, wp_users.user_nicename
        UNION
        SELECT wp_users.ID, wp_users.user_nicename, MAX(time) as last_message, (SELECT meta_value FROM wp_usermeta WHERE user_id = sender_user_id AND meta_key = 'uid') AS uid FROM `wp_usermessages`
            LEFT JOIN wp_users ON wp_users.ID = sender_user_id
            WHERE recipient_user_id = 1
            GROUP BY sender_user_id, wp_users.user_nicename
        ) SentReceived
     ORDER BY last_message) SentReceivedOrdered
GROUP BY ID, user_nicename, uid
ORDER BY `last_message`

提琴

db在这里摆弄：https://www.db-fiddle.com/f/6z7jVN6DtUYcuZwSXxeMX7/0

样本数据：

CREATE TABLE `wp_usermessages` (
  `id` bigint(20) NOT NULL,
  `time` datetime NOT NULL,
  `sender_user_id` bigint(20) UNSIGNED NOT NULL,
  `recipient_user_id` bigint(20) UNSIGNED NOT NULL,
  `message` text NOT NULL,
  `message_read` bit(1) NOT NULL DEFAULT b'0');

INSERT INTO `wp_usermessages` (`id`, `time`, `sender_user_id`, `recipient_user_id`, `message`, `message_read`) VALUES
(1, '2019-02-26 08:29:57', 2, 1, 'Hi, just wanted to check how you are?', b'0'),
(2, '2019-02-28 08:29:24', 1, 2, 'good thank you', b'0'),
(2, '2019-03-28 08:29:24', 1, 2, 'already been a month', b'0'),
(2, '2019-02-28 08:29:24', 1, 2, 'good thank you', b'0'),
(3, '2019-02-28 08:30:05', 5, 1, 'hi', b'1'),
(3, '2019-02-28 08:00:00', 5, 1, 'anyone', b'1'),
(3, '2019-02-28 08:05:00', 5, 1, 'hello', b'1');


CREATE TABLE `wp_usermeta` (
  `umeta_id` bigint(20) UNSIGNED NOT NULL,
  `user_id` bigint(20) UNSIGNED NOT NULL DEFAULT '0',
  `meta_key` varchar(255) DEFAULT NULL,
  `meta_value` longtext
);

INSERT INTO `wp_usermeta` (`umeta_id`, `user_id`, `meta_key`, `meta_value`) VALUES
(73, 1, 'uid', '9f39aa0ecd89d45e'),
(74, 5, 'uid', '0000000000000000'),
(75, 2, 'uid', '1212121212121212');

CREATE TABLE `wp_users` (
  `ID` bigint(20) UNSIGNED NOT NULL,
  `user_login` varchar(60) NOT NULL DEFAULT '',
  `user_pass` varchar(255) NOT NULL DEFAULT '',
  `user_nicename` varchar(50) NOT NULL DEFAULT '',
  `user_email` varchar(100) NOT NULL DEFAULT '',
  `user_url` varchar(100) NOT NULL DEFAULT '',
  `user_registered` datetime NOT NULL,
  `user_activation_key` varchar(255) NOT NULL DEFAULT '',
  `user_status` int(11) NOT NULL DEFAULT '0',
  `display_name` varchar(250) NOT NULL DEFAULT ''
);

INSERT INTO `wp_users` (`ID`, `user_login`, `user_pass`, `user_nicename`, `user_email`, `user_url`, `user_registered`, `user_activation_key`, `user_status`, `display_name`) VALUES
(1, 'ryan', '-', 'ryan', '-', '', '2018-12-05 08:13:11', '', 0, 'ryan'),
(2, 'jim', '-', 'jim', '-', '', '2019-02-01 12:43:29', '', 0, 'test'),
(5, 'bob', '-', 'bob', '-', '', '2019-02-02 09:17:54', '', 0, 'test2');

Answer 1

假设您从发布的查询中获得了正确的结果，这是一种更简单的方法。请注意代码本身的注释：

SELECT `ID`, `user_nicename`, `meta_value` as `uid`, `last_message`
FROM 
(
    -- Get the last time a message was sent between any user to user 1 
    SELECT `user_id`, MAX(`time`) AS last_message
    FROM (
        -- Get all the messages where user 1 was either the sender or the recipient
        -- with the message time.
        -- Using case to get the other user id for each record.
        SELECT  CASE WHEN `sender_user_id` 1 THEN `recipient_user_id` 
                ELSE `sender_user_id` 
                END As `user_id`, 
                `time`
        FROM `wp_usermessages`
        WHERE 1 IN(`recipient_user_id`, `sender_user_id`)
    ) AS A
    GROUP BY `user_id`
) AS B 
JOIN
(
    -- Get all the other details of the users
    SELECT `ID`, `user_nicename`, `meta_value` 
    FROM `wp_users` 
    JOIN `wp_usermeta` 
    ON `ID` = `user_id`
) AS C 
    ON B.user_id = C.ID

Answer 2

另一种可能且更简单的方法是

SELECT
    tbl.conv_with,
    MAX(tbl.last_message),
    u.user_nicename,
    meta.meta_value
FROM (
        -- select the latest message for each chat
        SELECT ( CASE
                    WHEN um.recipient_user_id = 1
                        THEN um.sender_user_id
                        ELSE um.recipient_user_id
                 END
               ) AS conv_with,
               um.time AS last_message
        FROM wp_usermessages um
        WHERE um.sender_user_id = 1 OR um.recipient_user_id = 1
    ) AS tbl
-- join wp_usermeta to get meta_data for message
LEFT JOIN wp_usermeta meta ON meta.user_id = tbl.conv_with
        AND meta.meta_key = 'uid'
-- join wp_users to get user_nicename for message
LEFT JOIN wp_users u ON u.id = tbl.conv_with
GROUP BY tbl.conv_with,u.user_nicename,meta.meta_value;

有没有更好的方法来简化许多嵌套的select语句？

提琴

2 个答案: