准备（两张表t1,t2）：

表t1:

mysql> select * from t1;

+-------+---------+

| t1_id | t1_name |

+-------+---------+

|     1 | t1_1    |

|     2 | t1_2    |

+-------+---------+

2 rows in set (0.02 sec)

表t2:

mysql> select * from t2;

+-------+---------+

| t2_id | t2_name |

+-------+---------+

|     2 | t2_2    |

|     3 | t2_3    |

+-------+---------+

2 rows in set (0.03 sec)

并集实例：

mysql> select t1.t1_id,t2.t2_id,t1.t1_name from t1 left join t2 on t1.t1_id=t2.t2_id;

+-------+-------+---------+

| t1_id | t2_id | t1_name |

+-------+-------+---------+

|     1 | NULL  | t1_1    |

|     2 |     2 | t1_2    |

+-------+-------+---------+

2 rows in set (0.02 sec)

交集实例：

mysql> select t1.t1_id,t2.t2_id,t1.t1_name from t1,t2 where t1.t1_id=t2.t2_id;

+-------+-------+---------+

| t1_id | t2_id | t1_name |

+-------+-------+---------+

|     2 |     2 | t1_2    |

+-------+-------+---------+

1 row in set (0.01 sec)