LEFT JOIN select latest row from the left join - mysql

Question

I have a company database that has all its details within it. It also has a separate table where credit ratings are stored.
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');
I am trying to only retrieve the company details and only the latest row on the credit rating table.
The SQL I have got closest to getting this to work is (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 FIDDLE HERE
At the moment it is bringing back 2 rows of the same company instead of just 1.
Thank you in advance for any pointers you guys can through my way.

Answer 1

prior versions don't have window functions but you can switch to user defined variables
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;
id | name | phone | islive | rating | thedate
--: | :--- | :---- | -----: | -----: | :------------------
190 | null | null | 1 | 222 | 2021-03-30 17:46:22
191 | null | null | 1 | null | null
db<>fiddle here

Answer 2

You can use a window function for this:
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;