准备数据

以一个简易问答系统为例，包括问题表和问题所属标签，问题表如下：

CREATE TABLE `t_qa` (  `id` bigint(20) NOT NULL AUTO_INCREMENT,  `title` varchar(200) NOT NULL DEFAULT '' COMMENT '标题',  `answer_count` int(5) unsigned NOT NULL DEFAULT '0' COMMENT '回答个数',  `label_id` bigint(20) unsigned NOT NULL DEFAULT '0' COMMENT '标签id',  `create_by` bigint(20) unsigned NOT NULL DEFAULT '0' COMMENT '创建人',  `create_date` datetime NOT NULL DEFAULT '0000-00-00 00:00:00' COMMENT '创建时间',  `update_by` bigint(20) unsigned DEFAULT NULL COMMENT '更新人',  `update_date` datetime DEFAULT NULL COMMENT '更新时间',  `del_flag` tinyint(1) unsigned NOT NULL DEFAULT '0' COMMENT '0：不删除，1：删除',  PRIMARY KEY (`id`)) ENGINE=InnoDB DEFAULT CHARSET=utf8;INSERT INTO `t_qa` (`id`, `title`, `answer_count`, `label_id`, `create_by`, `create_date`, `update_by`, `update_date`, `del_flag`)VALUES    (1, 'Java是什么？', 5, 1, 0, '2017-08-24 17:43:53', 0, '2017-08-24 17:43:53', 0),    (2, 'PHP是什么？', 4, 2, 0, '2017-08-24 17:43:53', 0, '2017-08-24 17:43:53', 0),    (3, '前端是什么？', 3, 3, 0, '2017-08-24 17:43:53', 0, '2017-08-24 17:43:53', 0),    (4, 'nodejs是什么？', 2, 0, 0, '2017-08-24 17:43:53', 0, '2017-08-24 17:43:53', 0),    (5, 'css是什么？', 1, 0, 0, '2017-08-24 17:43:53', 0, '2017-08-24 17:43:53', 0),    (6, 'JavaScript是什么？', 0, 0, 0, '2017-08-24 17:43:53', 0, '2017-08-24 17:43:53', 0);

标签表如下：

CREATE TABLE `t_label` (  `id` bigint(20) NOT NULL AUTO_INCREMENT,  `name` varchar(50) NOT NULL DEFAULT '' COMMENT '名称',  `create_by` bigint(20) unsigned NOT NULL DEFAULT '0' COMMENT '创建人',  `create_date` datetime NOT NULL DEFAULT '0000-00-00 00:00:00' COMMENT '创建时间',  `update_by` bigint(20) unsigned DEFAULT NULL COMMENT '更新人',  `update_date` datetime DEFAULT NULL COMMENT '更新时间',  `del_flag` tinyint(1) unsigned NOT NULL DEFAULT '0' COMMENT '0：不删除，1：删除',  PRIMARY KEY (`id`)) ENGINE=InnoDB DEFAULT CHARSET=utf8;INSERT INTO `t_label` (`id`, `name`, `create_by`, `create_date`, `update_by`, `update_date`, `del_flag`)VALUES    (1, 'java', 0, '2017-08-24 17:43:53', 0, '2017-08-24 17:43:53', 0),    (2, 'php', 0, '2017-08-24 17:43:53', 0, '2017-08-24 17:43:53', 0),    (3, '大前端', 0, '2017-08-24 17:43:53', 0, '2017-08-24 17:43:53', 0),    (4, 'mybatis', 0, '2017-08-24 17:43:53', 0, '2017-08-24 17:43:53', 0),    (5, 'python', 0, '2017-08-24 17:43:53', 0, '2017-08-24 17:43:53', 0),    (6, '多线程', 0, '2017-08-24 17:43:53', 0, '2017-08-24 17:43:53', 0);

一、左连接（LEFT JOIN）

SELECT     tq.title, tq.answer_count, tl.id, tl.nameFROM     t_qa tq LEFT JOIN t_label tl ON tq.label_id = tl.id

二、右连接（RIGHT JOIN）

SELECT     tq.title, tq.answer_count, tl.id, tl.nameFROM     t_qa tq RIGHT JOIN t_label tl ON tq.label_id = tl.id

三、内连接（INNER JOIN）

SELECT     tq.title, tq.answer_count, tl.id, tl.nameFROM     t_qa tq INNER JOIN t_label tl ON tq.label_id = tl.id

四、左独有连接（LEFT JOIN）

SELECT     tq.title, tq.answer_count, tl.id, tl.nameFROM     t_qa tq LEFT JOIN t_label tl ON tq.label_id = tl.idWHERE    tl.id IS NULL

五、右独有连接（RIGHT JOIN）

SELECT     tq.title, tq.answer_count, tl.id, tl.nameFROM     t_qa tq RIGHT JOIN t_label tl ON tq.label_id = tl.idWHERE    tq.label_id IS NULL

六、全连接（FULL JOIN）

由于MySQL不支持FULL OUTER JOIN，所以如果有全连接需求时，可用表达式：full outer join = left outer join UNION right outer join来实现。

SELECT tq.title, tq.answer_count, tl.id, tl.name FROM t_qa tq LEFT JOIN t_label tl ON tq.label_id = tl.idUNIONSELECT tq.title, tq.answer_count, tl.id, tl.name FROM t_qa tq RIGHT JOIN t_label tl ON tq.label_id = tl.id

七、全连接去交集（FULL JOIN）

SELECT tq.title, tq.answer_count, tl.id, tl.name FROM t_qa tq LEFT JOIN t_label tl ON tq.label_id = tl.id WHERE tl.id IS NULLUNIONSELECT tq.title, tq.answer_count, tl.id, tl.name FROM t_qa tq RIGHT JOIN t_label tl ON tq.label_id = tl.id WHERE tq.label_id IS NULL