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Mysql有很多种连接方式，其中有两大主要连接方式：内连接和外连接，其中外连接又可以分为左外连接和右外连接；
对于这三种连接方式，如何判断何时使用一直困扰着我，这里就做一下自己的总结：
1、内连接：INNER JOIN，通常用来连接两个表中都存在的列值，在数学上可以理解为两个数的交集，即共同拥有的部分；
例如：student(id,name,age,sex,score),teacher(id,name,age,sex,salary)
测试语句：
SELECT * FROM student s INNER JOIN teacher t ；
测试结果：
2、外连接：
左连接：LEFT JOIN，会将语句左边的表中的所有记录全部表示出来，而右表只会显示符合搜索条件的记录，如果左表的记录右表不存在，则以NULL来填充；
例如：student(id,name,age,sex,score),teacher(id,name,age,sex,salary)
测试语句：SELECT * FROM student s LEFT JOIN teacher t on s.id = t.id
测试结果：
右连接：RIGHT JOIN，会将语句右边的表中的所有记录全部表示出来，而左表只会显示符合搜索条件的记录，如果右表的记录左表不存在，则以NULL来填充；
例如：student(id,name,age,sex,score),teacher(id,name,age,sex,salary)
测试语句：SELECT * FROM student s RIght JOIN teacher t on s.id = t.id
测试结果：
由此可见，内连接的区别相当于数学中的交集，外连接的左右区分为：左连接为左全查右，右连接为右全查左！
