LEFT JOIN select 最新行从左加入

Question

我有一个公司数据库，其中包含所有详细信息。 它还有一个单独的表，用于存储信用评级。

DROP TABLE IF EXISTS `company`;
CREATE TABLE IF NOT EXISTS `company` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) CHARACTER SET latin1 NOT NULL,
  `email` varchar(255) CHARACTER SET latin1 NOT NULL,
  `address` varchar(255) NOT NULL,
  `address2` text NOT NULL,
  `address3` text NOT NULL,
  `phone` text NOT NULL,
  `contacts` text NOT NULL,
  `islive` tinyint(1) NOT NULL DEFAULT '1',
  `qt` tinyint(1) NOT NULL DEFAULT '30',
  `pt` tinyint(1) NOT NULL DEFAULT '30',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

DROP TABLE IF EXISTS `creditrating`;
CREATE TABLE IF NOT EXISTS `creditrating` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `cid` int(11) NOT NULL,
  `rating` int(11) NOT NULL,
  `arating` varchar(10) NOT NULL,
  `type` tinyint(1) NOT NULL DEFAULT '1',
  `thedate` datetime NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

INSERT INTO `company` (`id`, `name`, `email`, `address`, `address2`, `address3`, `phone`, `contacts`, `islive`, `qt`, `pt`) VALUES
(190, 'Test Company', 'test@test.com', 'Testing Client 1\r\nsdfsdfsdf\r\nsdfsdfsdf', 'Testing Client 1\r\nsdfsdfsdf\r\nsdfsdfsdf', 'Testing Client 1\r\nsdfsdfsdf\r\nsdfsdfsdf', '65165156', 'test name', 1, 30, 30),
(191, 'Test Company 2', 'test2@test2.com', 'Testing Client 2\r\nsdfsdfsdf\r\nsdfsdfsdf', 'Testing Client 2\r\nsdfsdfsdf\r\nsdfsdfsdf', 'Testing Client 2\r\nsdfsdfsdf\r\nsdfsdfsdf', '65165156', 'test name 2', 1, 30, 30);

INSERT INTO `creditrating` (`id`, `cid`, `rating`, `arating`, `type`, `thedate`) VALUES
(3, 190, 684, 'da774', 1, '2021-03-30 15:08:52'),
(6, 190, 222, 'DD222', 1, '2021-03-30 17:46:22');

我试图只检索公司详细信息和信用评级表上的最新行。

我最接近让它工作的 SQL 是（Mysql 5.6）：

SELECT c.id
     , AES_DECRYPT(c.name, 'co1') as name
     , AES_DECRYPT(c.phone, 'co3') as phone
     , c.islive
     , r.rating 
  FROM company c 
  LEFT 
  JOIN creditrating r 
    ON (SELECT r.thedate FROM creditrating WHERE c.id=r.cid ORDER BY r.thedate DESC LIMIT 1)

DB 小提琴在这里

目前，它正在带回同一家公司的 2 排而不是 1 排。

预先感谢您通过我的方式提供的任何指示。

Answer 1

您可以为此使用 window function ：

SELECT c.id, AES_DECRYPT(c.name, 'co1') as name, AES_DECRYPT(c.phone, 'co3') as phone, c.islive, r.rating
FROM company c LEFT JOIN
     (SELECT r.*,
              ROW_NUMBER() OVER (PARTITION BY r.cid ORDER BY r.thedate DESC) as seqnum
      FROM creditrating r
     ) r
     ON c.id = r.cid AND r.seqnum = 1;

Answer 2

以前的版本没有 window 功能，但您可以切换到用户定义的变量

DROP TABLE IF EXISTS `company`; CREATE TABLE IF NOT EXISTS `company` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(255) CHARACTER SET latin1 NOT NULL, `email` varchar(255) CHARACTER SET latin1 NOT NULL, `address` varchar(255) NOT NULL, `address2` text NOT NULL, `address3` text NOT NULL, `phone` text NOT NULL, `contacts` text NOT NULL, `islive` tinyint(1) NOT NULL DEFAULT '1', `qt` tinyint(1) NOT NULL DEFAULT '30', `pt` tinyint(1) NOT NULL DEFAULT '30', PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; DROP TABLE IF EXISTS `creditrating`; CREATE TABLE IF NOT EXISTS `creditrating` ( `id` int(11) NOT NULL AUTO_INCREMENT, `cid` int(11) NOT NULL, `rating` int(11) NOT NULL, `arating` varchar(10) NOT NULL, `type` tinyint(1) NOT NULL DEFAULT '1', `thedate` datetime NOT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=latin1; INSERT INTO `company` (`id`, `name`, `email`, `address`, `address2`, `address3`, `phone`, `contacts`, `islive`, `qt`, `pt`) VALUES (190, 'Test Company', 'test@test.com', 'Testing Client 1\r\nsdfsdfsdf\r\nsdfsdfsdf', 'Testing Client 1\r\nsdfsdfsdf\r\nsdfsdfsdf', 'Testing Client 1\r\nsdfsdfsdf\r\nsdfsdfsdf', '65165156', 'test name', 1, 30, 30), (191, 'Test Company 2', 'test2@test2.com', 'Testing Client 2\r\nsdfsdfsdf\r\nsdfsdfsdf', 'Testing Client 2\r\nsdfsdfsdf\r\nsdfsdfsdf', 'Testing Client 2\r\nsdfsdfsdf\r\nsdfsdfsdf', '65165156', 'test name 2', 1, 30, 30); INSERT INTO `creditrating` (`id`, `cid`, `rating`, `arating`, `type`, `thedate`) VALUES (3, 190, 684, 'da774', 1, '2021-03-30 15:08:52'), (6, 190, 222, 'DD222', 1, '2021-03-30 17:46:22');

 SELECT id, name, phone, islive, rating, `thedate` FROM (SELECT name, phone, islive, rating, `thedate`, IF(id = @id, @rnk:=@rnk + 1, @rnk:=1) AS rnk, @id:=id AS id FROM (SELECT c.id, AES_DECRYPT(c.name, 'co1') AS name, AES_DECRYPT(c.phone, 'co3') AS phone, c.islive, r.rating, `thedate` FROM company c LEFT JOIN creditrating r ON c.id = r.cid) t1, (SELECT @id:=- 1, @rnk:=0) t2 ORDER BY id, `thedate` DESC) t3 WHERE rnk = 1;

 编号 | 姓名 | 电话 | 生活 | 评级 | 日期 --: |:--- |:---- |  -----: |  -----: |:----------------- 190 |  null |  null |  1 |  222 |  2021-03-30 17:46:22 191 |  null |  null |  1 |  null |  null

db<> 在这里摆弄