MySQL表中的连接方式其实非常简单，这里就简单的罗列出他们的特点。
表的连接（JOIN）可以分为内连接（JOIN/INNER JOIN）和外连接（LEFT JOIN/RIGHT JOIN）。
首先我们看一下我们本次演示的两个表：
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mysql> SELECT * FROM student;+------+----------+------+------+| s_id | s_name | age | c_id |+------+----------+------+------+| 1 | xiaoming | 13 | 1 || 2 | xiaohong | 41 | 4 || 3 | xiaoxia | 22 | 3 || 4 | xiaogang | 32 | 1 || 5 | xiaoli | 41 | 2 || 6 | wangwu | 13 | 2 || 7 | lisi | 22 | 3 || 8 | zhangsan | 11 | 9 |+------+----------+------+------+8 rows in set (0.00 sec)mysql> SELECT * FROM class;+------+---------+-------+| c_id | c_name | count |+------+---------+-------+| 1 | MATH | 65 || 2 | CHINESE | 70 || 3 | ENGLISH | 50 || 4 | HISTORY | 30 || 5 | BIOLOGY | 40 |+------+---------+-------+5 rows in set (0.00 sec) |
首先，表要能连接的前提就是两个表中有相同的可以比较的列。
1.内连接
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mysql> SELECT * FROM student INNER JOIN class ON student.c_id = class.c_id;+------+----------+------+------+------+---------+-------+| s_id | s_name | age | c_id | c_id | c_name | count |+------+----------+------+------+------+---------+-------+| 1 | xiaoming | 13 | 1 | 1 | MATH | 65 || 2 | xiaohong | 41 | 4 | 4 | HISTORY | 30 || 3 | xiaoxia | 22 | 3 | 3 | ENGLISH | 50 || 4 | xiaogang | 32 | 1 | 1 | MATH | 65 || 5 | xiaoli | 41 | 2 | 2 | CHINESE | 70 || 6 | wangwu | 13 | 2 | 2 | CHINESE | 70 || 7 | lisi | 22 | 3 | 3 | ENGLISH | 50 |+------+----------+------+------+------+---------+-------+7 rows in set (0.00 sec) |
简单的讲，内连接就是把两个表中符合条件的行的所有数据一起展示出来，即如果不符合条件，即在表A中找得到但是在B中没有（或者相反）的数据不予以显示。
2.外连接
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mysql> SELECT * FROM student LEFT JOIN class ON student.c_id = class.c_id;+------+----------+------+------+------+---------+-------+| s_id | s_name | age | c_id | c_id | c_name | count |+------+----------+------+------+------+---------+-------+| 1 | xiaoming | 13 | 1 | 1 | MATH | 65 || 2 | xiaohong | 41 | 4 | 4 | HISTORY | 30 || 3 | xiaoxia | 22 | 3 | 3 | ENGLISH | 50 || 4 | xiaogang | 32 | 1 | 1 | MATH | 65 || 5 | xiaoli | 41 | 2 | 2 | CHINESE | 70 || 6 | wangwu | 13 | 2 | 2 | CHINESE | 70 || 7 | lisi | 22 | 3 | 3 | ENGLISH | 50 || 8 | zhangsan | 11 | 9 | NULL | NULL | NULL |+------+----------+------+------+------+---------+-------+8 rows in set (0.00 sec)mysql> SELECT * FROM student RIGHT JOIN class ON student.c_id = class.c_id;+------+----------+------+------+------+---------+-------+| s_id | s_name | age | c_id | c_id | c_name | count |+------+----------+------+------+------+---------+-------+| 1 | xiaoming | 13 | 1 | 1 | MATH | 65 || 4 | xiaogang | 32 | 1 | 1 | MATH | 65 || 5 | xiaoli | 41 | 2 | 2 | CHINESE | 70 || 6 | wangwu | 13 | 2 | 2 | CHINESE | 70 || 3 | xiaoxia | 22 | 3 | 3 | ENGLISH | 50 || 7 | lisi | 22 | 3 | 3 | ENGLISH | 50 || 2 | xiaohong | 41 | 4 | 4 | HISTORY | 30 || NULL | NULL | NULL | NULL | 5 | BIOLOGY | 40 |+------+----------+------+------+------+---------+-------+8 rows in set (0.00 sec) |
上面分别展示了外连接的两种情况：左连接和右连接。这两种几乎是一样的，唯一的区别就是左连接的主表是左边的表，右连接的主表是右边的表。而外连接与内连接不同的地方就是它会将主表的所有行都予以显示，而在主表中有，其他表中没有的数据用NULL代替。
总结
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原文链接：https://segmentfault.com/a/1190000038203698
