mysql 左联结与右联结

mysql> select * from test;
+----+------------+-------+-----------+
| id | name       | score | subject   |
+----+------------+-------+-----------+
|   | xiaoming   |     | shuxue    |
|   | xiaohong   |     | shuxue    |
|   | xiaohong   |     | english   |
|   | xiaohong   |     | physics   |
|   | xiaohong   |     | astronaut |
|   | xiaoming   |     | physics   |
|   | xiaoming   |     | astronaut |
|   | xiaoming   |     | english   |
|   | xiaobai    |     | astronaut |
|  | .2xiaobai |     | english   |
|  | .2xiaobai |     | physics   |
|  | 3xiaobai   |     | shuxue    |
|  | 123xiaohei |     | astronaut |
|  | xiaohei    |     | shuxue    |
|  | xiaohei    |     | physics   |
|  | .12xiaohei |     | english   |
+----+------------+-------+-----------+
 rows in set (0.00 sec)

mysql> select * from user;
+------+----------+-----------+
| id   | name     | address   |
+------+----------+-----------+
|     | xiaoming | beijing   |
|     | xiaobai  | shandong  |
|     | xiaohong | suzhou    |
|     | xiaohei  | changchun |
+------+----------+-----------+
 rows in set (0.00 sec)

mysql> select * from user left join test on user.name=test.name;
+------+----------+-----------+------+----------+-------+-----------+
| id   | name     | address   | id   | name     | score | subject   |
+------+----------+-----------+------+----------+-------+-----------+
|     | xiaoming | beijing   |     | xiaoming |     | shuxue    |
|     | xiaohong | suzhou    |     | xiaohong |     | shuxue    |
|     | xiaohong | suzhou    |     | xiaohong |     | english   |
|     | xiaohong | suzhou    |     | xiaohong |     | physics   |
|     | xiaohong | suzhou    |     | xiaohong |     | astronaut |
|     | xiaoming | beijing   |     | xiaoming |     | physics   |
|     | xiaoming | beijing   |     | xiaoming |     | astronaut |
|     | xiaoming | beijing   |     | xiaoming |     | english   |
|     | xiaobai  | shandong  |     | xiaobai  |     | astronaut |
|     | xiaohei  | changchun |    | xiaohei  |     | shuxue    |
|     | xiaohei  | changchun |    | xiaohei  |     | physics   |
+------+----------+-----------+------+----------+-------+-----------+
 rows in set (0.00 sec)

mysql> select * from user right join test on user.name=test.name;
+------+----------+-----------+----+------------+-------+-----------+
| id   | name     | address   | id | name       | score | subject   |
+------+----------+-----------+----+------------+-------+-----------+
|     | xiaoming | beijing   |   | xiaoming   |     | shuxue    |
|     | xiaoming | beijing   |   | xiaoming   |     | physics   |
|     | xiaoming | beijing   |   | xiaoming   |     | astronaut |
|     | xiaoming | beijing   |   | xiaoming   |     | english   |
|     | xiaobai  | shandong  |   | xiaobai    |     | astronaut |
|     | xiaohong | suzhou    |   | xiaohong   |     | shuxue    |
|     | xiaohong | suzhou    |   | xiaohong   |     | english   |
|     | xiaohong | suzhou    |   | xiaohong   |     | physics   |
|     | xiaohong | suzhou    |   | xiaohong   |     | astronaut |
|     | xiaohei  | changchun |  | xiaohei    |     | shuxue    |
|     | xiaohei  | changchun |  | xiaohei    |     | physics   |
| NULL | NULL     | NULL      |  | .2xiaobai |     | english   |
| NULL | NULL     | NULL      |  | .2xiaobai |     | physics   |
| NULL | NULL     | NULL      |  | 3xiaobai   |     | shuxue    |
| NULL | NULL     | NULL      |  | 123xiaohei |     | astronaut |
| NULL | NULL     | NULL      |  | .12xiaohei |     | english   |
+------+----------+-----------+----+------------+-------+-----------+
 rows in set (0.00 sec)