MySQL的几种表的连接方式

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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：

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

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

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。

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