MySQL表中的連接方式其實(shí)非常簡單，這里就簡單的羅列出他們的特點(diǎn)。
表的連接（JOIN）可以分為內(nèi)連接（JOIN/INNER JOIN）和外連接（LEFT JOIN/RIGHT JOIN）。
首先我們看一下我們本次演示的兩個表：
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，表要能連接的前提就是兩個表中有相同的可以比較的列。
1.內(nèi)連接
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)
簡單的講，內(nèi)連接就是把兩個表中符合條件的行的所有數(shù)據(jù)一起展示出來，即如果不符合條件，即在表A中找得到但是在B中沒有（或者相反）的數(shù)據(jù)不予以顯示。
2.外連接
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)
上面分別展示了外連接的兩種情況：左連接和右連接。這兩種幾乎是一樣的，唯一的區(qū)別就是左連接的主表是左邊的表，右連接的主表是右邊的表。而外連接與內(nèi)連接不同的地方就是它會將主表的所有行都予以顯示，而在主表中有，其他表中沒有的數(shù)據(jù)用NULL代替。
總結(jié)
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