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)
mysql查询，left join（求并集），where（求交集）
原文：http://12173069.blog.51cto.com/12163069/1968871