SQL join中on和where的区别解析
<div id="navCategory"><h5 class="catalogue">目录</h5><ul class="first_class_ul"><li><a href="#_label0">0.结论</a></li><li><a href="#_label1">1.数据准备</a></li><li><a href="#_label2">2.测试</a></li><ul class="second_class_ul"><li><a href="#_lab2_2_0">2.1.普通</a></li><ul class="third_class_ul"><li><a href="#_label3_2_0_0">2.1.1.class单表</a></li><li><a href="#_label3_2_0_1">2.1.2.student单表</a></li><li><a href="#_label3_2_0_2">2.1.3.笛卡尔积</a></li><li><a href="#_label3_2_0_3">2.1.4. 内连接</a></li><li><a href="#_label3_2_0_4">2.2.5.普通外连</a></li></ul><li><a href="#_lab2_2_1">2.2.重点来啦-外连接</a></li><ul class="third_class_ul"><li><a href="#_label3_2_1_5">2.2.1.一</a></li><li><a href="#_label3_2_1_6">2.2.2.二</a></li><li><a href="#_label3_2_1_7">2.2.3.三</a></li><li><a href="#_label3_2_1_8">2.2.4.四</a></li></ul></ul><li><a href="#_label3">3.参考资料</a></li><ul class="second_class_ul"></ul></ul></div><p class="maodian"><a name="_label0"></a></p><h2>0.结论</h2><ul><li>两个表在，join时，首先做一个笛卡尔积，on后面的条件是对这个笛卡尔积做一个过滤形成一张临时表，如果没有where就直接返回结果，如果有where就对上一步的临时表再进行过滤。<ul><li>先on，再join，再where</li><li>在使用left join时，on和where条件的区别如下：</li></ul></li><li>1、on条件是在生成临时表时使用的条件，它不管on中的条件是否为真，都会返回左边表中的记录。</li><li>2、where条件是在临时表生成好后，再对临时表进行过滤的条件。这时已经没有left join的含义（必须返回左边表的记录）了，条件不为真的就全部过滤掉</li></ul>
<p class="maodian"><a name="_label1"></a></p><h2>1.数据准备</h2>
<div class="jb51code"><pre class="brush:sql;">DROP TABLE IF EXISTS `class`;
CREATE TABLE `class`
(
`c_id` int DEFAULT NULL COMMENT '班级ID',
`c_name` varchar(50) DEFAULT NULL COMMENT '班级名'
);
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student`
(
`s_id` int DEFAULT NULL COMMENT '学生ID',
`s_name` varchar(50) DEFAULT NULL COMMENT '学生名',
`c_id` int DEFAULT NULL COMMENT '班级ID'
);
INSERT INTO `class` (`c_id`, `c_name`)
VALUES (1, '一班'),
(2, '二班'),
(3, '三班');
INSERT INTO `student` (`s_id`, `s_name`, `c_id`)
VALUES (1, '张三', 1),
(2, '李四', 2),
(3, '王五', 4);</pre></div>
<p class="maodian"><a name="_label2"></a></p><h2>2.测试</h2>
<div class="jb51code"><pre class="brush:sql;">-- 单表
select * from class c;
select * from student s;
-- 笛卡尔积
select * from class c inner join student s order by c.c_id, s.s_id;
-- 内连
select * from class c inner join student s on c.c_id = s.c_id order by c.c_id, s.s_id;
-- 左外连（先on，再join，再where）
select * from class c leftjoin student s on c.c_id = s.c_id order by c.c_id, s.s_id;
select * from class c leftjoin student s on c.c_id = s.c_id and c.c_id <> 2 order by c.c_id, s.s_id;
select * from class c leftjoin student s on c.c_id = s.c_id where c.c_id <> 2 order by c.c_id, s.s_id;
select * from class c leftjoin student s on c.c_id = s.c_id and s.c_id <> 2 order by c.c_id, s.s_id;
select * from class c leftjoin student s on c.c_id = s.c_id where s.c_id <> 2 order by c.c_id, s.s_id;</pre></div>
<p class="maodian"><a name="_lab2_2_0"></a></p><h3>2.1.普通</h3>
<p class="maodian"><a name="_label3_2_0_0"></a></p><h4>2.1.1.class单表</h4>
<p style="text-align:center"><img alt="" src="https://img.jbzj.com/file_images/article/202508/2025081109380928.png" /></p>
<p class="maodian"><a name="_label3_2_0_1"></a></p><h4>2.1.2.student单表</h4>
<p style="text-align:center"><img alt="" src="https://img.jbzj.com/file_images/article/202508/2025081109380964.png" /></p>
<p class="maodian"><a name="_label3_2_0_2"></a></p><h4>2.1.3.笛卡尔积</h4>
<p style="text-align:center"><img alt="" src="https://img.jbzj.com/file_images/article/202508/2025081109380971.png" /></p>
<p class="maodian"><a name="_label3_2_0_3"></a></p><h4>2.1.4. 内连接</h4>
<p style="text-align:center"><img alt="" src="https://img.jbzj.com/file_images/article/202508/2025081109380924.png" /></p>
<p class="maodian"><a name="_label3_2_0_4"></a></p><h4>2.2.5.普通外连</h4>
<p style="text-align:center"><img alt="" src="https://img.jbzj.com/file_images/article/202508/2025081109380998.png" /></p>
<p class="maodian"><a name="_lab2_2_1"></a></p><h3>2.2.重点来啦-外连接</h3>
<p class="maodian"><a name="_label3_2_1_5"></a></p><h4>2.2.1.一</h4>
<p style="text-align:center"><img alt="" src="https://img.jbzj.com/file_images/article/202508/2025081109380985.png" /></p>
<p class="maodian"><a name="_label3_2_1_6"></a></p><h4>2.2.2.二</h4>
<p style="text-align:center"><img alt="" src="https://img.jbzj.com/file_images/article/202508/2025081109380922.png" /></p>
<p class="maodian"><a name="_label3_2_1_7"></a></p><h4>2.2.3.三</h4>
<p style="text-align:center"><img alt="" src="https://img.jbzj.com/file_images/article/202508/2025081109380979.png" /></p>
<p class="maodian"><a name="_label3_2_1_8"></a></p><h4>2.2.4.四</h4>
<p style="text-align:center"><img alt="" src="https://img.jbzj.com/file_images/article/202508/2025081109380987.png" /></p>
<p class="maodian"><a name="_label3"></a></p><h2>3.参考资料</h2>
<p><a href="https://www.jb51.net/article/270224.htm" rel="external nofollow" target="_blank">SQL语句中LEFT JOIN的ON和WHERE有什么区别</a></p>
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