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。