关于mysql:MySQL的几种表的连接方式

MySQL 表中的连贯形式其实非常简单，这里就简略的列举出他们的特点。
表的连贯（JOIN）能够分为内连贯（JOIN/INNER JOIN）和外连贯（LEFT JOIN/RIGHT JOIN）。

首先咱们看一下咱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，表要能连贯的前提就是两个表中有雷同的能够比拟的列。

1. 内连贯

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)

简略的讲，内连贯就是把两个表中符合条件的行的所有数据一起展现进去，即如果不符合条件，即在表 A 中找失去然而在 B 中没有（或者相同）的数据不予以显示。

2. 外连贯

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)

下面别离展现了外连贯的两种状况：左连贯和右连贯。这两种简直是一样的，惟一的区别就是左连贯的主表是右边的表，右连贯的主表是左边的表。而外连贯与内连贯不同的中央就是它会将主表的所有行都予以显示，而在主表中有，其余表中没有的数据用 NULL 代替。