准备测试表tb1和tb2并插入数据http://www.cndba.cn/hbhe0316/article/22644

mysql> select * from tb1;
+------+
| id   |
+------+
|    1 |
|    2 |
|    3 |
|    4 |
|    5 |
+------+
5 rows in set (0.00 sec)

mysql> select * from tb2;
+------+
| id   |
+------+
|    4 |
|    5 |
|    6 |
|    7 |
|    8 |
+------+
5 rows in set (0.00 sec)

1.左外部链接：匹配A、B两个数据集，仅返回A中存在的数据。
http://www.cndba.cn/hbhe0316/article/22644http://www.cndba.cn/hbhe0316/article/22644

mysql> select * from tb1 left join tb2 on tb1.id=tb2.id;
+------+------+
| id   | id   |
+------+------+
|    4 |    4 |
|    5 |    5 |
|    1 | NULL |
|    2 | NULL |
|    3 | NULL |
+------+------+
5 rows in set (0.00 sec)

2.右外部连接：匹配A、B两个数据集，仅返回B中存在的数据。
http://www.cndba.cn/hbhe0316/article/22644

mysql> select * from tb1 right join tb2 on tb1.id=tb2.id;
+------+------+
| id   | id   |
+------+------+
|    4 |    4 |
|    5 |    5 |
| NULL |    6 |
| NULL |    7 |
| NULL |    8 |
+------+------+
5 rows in set (0.00 sec)

3.内部连接：匹配A、B两个数据集，仅返回A、B均存在的数据。
http://www.cndba.cn/hbhe0316/article/22644http://www.cndba.cn/hbhe0316/article/22644http://www.cndba.cn/hbhe0316/article/22644http://www.cndba.cn/hbhe0316/article/22644

mysql> select * from tb1 inner join tb2 on tb1.id=tb2.id;
+------+------+
| id   | id   |
+------+------+
|    4 |    4 |
|    5 |    5 |
+------+------+
2 rows in set (0.00 sec)

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MYSQL