Example analysis of the use of GROUP_CONCAT in MySQL

CREATE TABLE `cate` (
 `id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT 'id',
 `name` char(20) DEFAULT '' COMMENT 'Category name',
 PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8 COMMENT='Article Category Table';

CREATE TABLE `article` (
 `id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT 'id',
 `title` varchar(50) DEFAULT '',
 `cate_id` int(11) NOT NULL DEFAULT '0' COMMENT 'Category id',
 PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8 COMMENT='Article table';

CREATE TABLE `article_extend` (
 `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
 `article_id` int(10) unsigned DEFAULT '0' COMMENT 'Article id',
 `name` varchar(255) DEFAULT '' COMMENT 'Audio, pictures, etc.',
 PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 COMMENT='Attachment table';

SELECT
	a.id AS aid,
	a.title AS atitle,
	c. NAME AS cname,
	ae.NAME AS aname
FROM
	article AS a
LEFT JOIN cate AS c ON a.cate_id = c.id
LEFT JOIN article_extend AS ae ON a.id = ae.article_id
WHERE
	a.id = 1;

SELECT
	a.id AS aid,
	a.title AS atitle,
	c. NAME AS cname,
	GROUP_CONCAT(ae.NAME SEPARATOR '-') AS aname
FROM
	article AS a
LEFT JOIN cate AS c ON a.cate_id = c.id
LEFT JOIN article_extend AS ae ON a.id = ae.article_id
WHERE
	a.id = 1;

SELECT
	a.id AS aid,
	a.title AS atitle,
	c. NAME AS cname,
	ae.allname
FROM
	article AS a
LEFT JOIN (
	SELECT
		ae.article_id,
		GROUP_CONCAT(ae.NAME) AS allname
	FROM
		article_extend AS ae
	GROUP BY
		ae.article_id
) AS ae ON a.id = ae.article_id
LEFT JOIN cate AS c ON a.cate_id = c.id;