来自多个表的 MySQL 最新相关记录

Question

假设一个主“作业”表和两个对应的“日志”表（一个用于服务器事件，另一个用于用户事件，每个表中存储的数据完全不同）。

从两个“日志”表（如果有的话）中返回选择的“作业”记录和最新的相应日志记录（具有多个字段）的最佳方式是什么。

确实从MySQL Order before Group by获得了一些灵感

以下 SQL 将创建一些示例表/数据...

CREATE TABLE job (
    `id` int(11) NOT NULL AUTO_INCREMENT,
    `name` tinytext NOT NULL,
    PRIMARY KEY (id)
);

CREATE TABLE job_log_server (
    `id` int(11) NOT NULL AUTO_INCREMENT,
    `job_id` int(11) NOT NULL,
    `event` tinytext NOT NULL,
    `ip` tinytext NOT NULL,
    `created` datetime NOT NULL,
    PRIMARY KEY (id),
    KEY job_id (job_id)
);

CREATE TABLE job_log_user (
    `id` int(11) NOT NULL AUTO_INCREMENT,
    `job_id` int(11) NOT NULL,
    `event` tinytext NOT NULL,
    `user_id` int(11) NOT NULL,
    `created` datetime NOT NULL,
    PRIMARY KEY (id),
    KEY job_id (job_id)
);

INSERT INTO job VALUES (1, 'Job A');
INSERT INTO job VALUES (2, 'Job B');
INSERT INTO job VALUES (3, 'Job C');
INSERT INTO job VALUES (4, 'Job D');

INSERT INTO job_log_server VALUES (1, 2, 'Job B Event 1', '127.0.0.1', '2000-01-01 00:00:01');
INSERT INTO job_log_server VALUES (2, 2, 'Job B Event 2', '127.0.0.1', '2000-01-01 00:00:02');
INSERT INTO job_log_server VALUES (3, 2, 'Job B Event 3*', '127.0.0.1', '2000-01-01 00:00:03');
INSERT INTO job_log_server VALUES (4, 3, 'Job C Event 1*', '127.0.0.1', '2000-01-01 00:00:04');

INSERT INTO job_log_user VALUES (1, 1, 'Job A Event 1', 5, '2000-01-01 00:00:01');
INSERT INTO job_log_user VALUES (2, 1, 'Job A Event 2*', 5, '2000-01-01 00:00:02');
INSERT INTO job_log_user VALUES (3, 2, 'Job B Event 1*', 5, '2000-01-01 00:00:03');
INSERT INTO job_log_user VALUES (4, 4, 'Job D Event 1', 5, '2000-01-01 00:00:04');
INSERT INTO job_log_user VALUES (5, 4, 'Job D Event 2', 5, '2000-01-01 00:00:05');
INSERT INTO job_log_user VALUES (6, 4, 'Job D Event 3*', 5, '2000-01-01 00:00:06');

一个选项（仅从每个表返回 1 个字段）是使用嵌套子查询...但 ORDER BY 必须在对 GROUP BY (x2) 的单独查询中完成：

SELECT
    *
FROM
    (
        SELECT
            s2.*,
            jlu.event AS user_event
        FROM
            (
                SELECT
                    *
                FROM
                    (
                        SELECT
                            j.id,
                            j.name,
                            jls.event AS server_event
                        FROM
                            job AS j
                        LEFT JOIN
                            job_log_server AS jls ON jls.job_id = j.id
                        ORDER BY
                            jls.created DESC
                    ) AS s1
                GROUP BY
                    s1.id
            ) AS s2
        LEFT JOIN
            job_log_user AS jlu ON jlu.job_id = s2.id
        ORDER BY
            jlu.created DESC
    ) AS s3
GROUP BY
    s3.id;

这实际上似乎表现得很好......只是不太容易理解。

或者您可以尝试在两个单独的子查询中返回和排序日志记录：

SELECT
    j.id,
    j.name,
    jls2.event AS server_event,
    jlu2.event AS user_event
FROM
    job AS j
LEFT JOIN
    (
        SELECT
            jls.job_id,
            jls.event
        FROM
            job_log_server AS jls
        ORDER BY
            jls.created DESC
    ) AS jls2 ON jls2.job_id = j.id
LEFT JOIN
    (
        SELECT
            jlu.job_id,
            jlu.event
        FROM
            job_log_user AS jlu
        ORDER BY
            jlu.created DESC
    ) AS jlu2 ON jlu2.job_id = j.id
GROUP BY
    j.id;

但这似乎需要更长的时间才能运行......可能是因为它添加到临时表中的记录数量，然后大部分被忽略（为了保持这个简短，我没有添加任何作业表的条件，否则只会返回活动作业）。

不确定我是否遗漏了任何明显的东西。

Answer 1

下面的SQL Fiddle怎么样。它产生的结果与您的两个查询相同。

SELECT j.id, j.name, 
( 
  SELECT s.event 
  FROM job_log_server s
  WHERE j.id = s.job_id
  ORDER BY s.id DESC
  LIMIT 1
)AS SERVER_EVENT,
( 
  SELECT u.event 
  FROM job_log_user u
  WHERE j.id = u.job_id
  ORDER BY u.id DESC
  LIMIT 1
)AS USER_EVENT
FROM job j

编辑 SQL Fiddle:

SELECT m.id, m.name, js.event AS SERVER_EVENT, ju.event AS USER_EVENT
FROM 
(
  SELECT j.id, j.name,
  ( 
    SELECT s.id
    FROM job_log_server s
    WHERE j.id = s.job_id
    ORDER BY s.id DESC
    LIMIT 1
  )AS S_E,
  ( 
    SELECT u.id
    FROM job_log_user u
    WHERE j.id = u.job_id
    ORDER BY u.id DESC
    LIMIT 1
  )AS U_E
  FROM job j
) m
LEFT JOIN job_log_server js ON js.id = m.S_E
LEFT JOIN job_log_user ju ON ju.id = m.U_E