mysql中GROUP_CONCAT的使用

现在有三个表，结构如下：

cate表
CREATE TABLE `cate` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT 'id',`name` char(20) DEFAULT '' COMMENT '分类名',PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8 COMMENT='文章分类表';

article表
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 '分类id',PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8 COMMENT='文章表';

article_extend表
CREATE TABLE `article_extend` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,`article_id` int(10) unsigned DEFAULT '0' COMMENT '文章id',`name` varchar(255) DEFAULT '' COMMENT '音频，图片之类',PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 COMMENT='附件表';

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,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,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;

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