mysql 左联结与右联结

mysql> select * from test;
+----+------------+-------+-----------+
| id | name       | score | subject   |
+----+------------+-------+-----------+
|  1 | xiaoming   |    89 | shuxue    |
|  2 | xiaohong   |    89 | shuxue    |
|  3 | xiaohong   |    80 | english   |
|  4 | xiaohong   |    80 | physics   |
|  5 | xiaohong   |    80 | astronaut |
|  6 | xiaoming   |    80 | physics   |
|  7 | xiaoming   |    80 | astronaut |
|  8 | xiaoming   |    80 | english   |
|  9 | xiaobai    |    80 | astronaut |
| 10 | 1.2xiaobai |    80 | english   |
| 11 | 2.2xiaobai |    80 | physics   |
| 12 | 3xiaobai   |    80 | shuxue    |
| 13 | 123xiaohei |    80 | astronaut |
| 14 | xiaohei    |    80 | shuxue    |
| 15 | xiaohei    |    80 | physics   |
| 16 | .12xiaohei |    80 | english   |
+----+------------+-------+-----------+
16 rows in set (0.00 sec)



mysql> select * from user;
+------+----------+-----------+
| id   | name     | address   |
+------+----------+-----------+
|    1 | xiaoming | beijing   |
|    2 | xiaobai  | shandong  |
|    3 | xiaohong | suzhou    |
|    4 | xiaohei  | changchun |
+------+----------+-----------+
4 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   |
+------+----------+-----------+------+----------+-------+-----------+
|    1 | xiaoming | beijing   |    1 | xiaoming |    89 | shuxue    |
|    3 | xiaohong | suzhou    |    2 | xiaohong |    89 | shuxue    |
|    3 | xiaohong | suzhou    |    3 | xiaohong |    80 | english   |
|    3 | xiaohong | suzhou    |    4 | xiaohong |    80 | physics   |
|    3 | xiaohong | suzhou    |    5 | xiaohong |    80 | astronaut |
|    1 | xiaoming | beijing   |    6 | xiaoming |    80 | physics   |
|    1 | xiaoming | beijing   |    7 | xiaoming |    80 | astronaut |
|    1 | xiaoming | beijing   |    8 | xiaoming |    80 | english   |
|    2 | xiaobai  | shandong  |    9 | xiaobai  |    80 | astronaut |
|    4 | xiaohei  | changchun |   14 | xiaohei  |    80 | shuxue    |
|    4 | xiaohei  | changchun |   15 | xiaohei  |    80 | physics   |
+------+----------+-----------+------+----------+-------+-----------+
11 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   |
+------+----------+-----------+----+------------+-------+-----------+
|    1 | xiaoming | beijing   |  1 | xiaoming   |    89 | shuxue    |
|    1 | xiaoming | beijing   |  6 | xiaoming   |    80 | physics   |
|    1 | xiaoming | beijing   |  7 | xiaoming   |    80 | astronaut |
|    1 | xiaoming | beijing   |  8 | xiaoming   |    80 | english   |
|    2 | xiaobai  | shandong  |  9 | xiaobai    |    80 | astronaut |
|    3 | xiaohong | suzhou    |  2 | xiaohong   |    89 | shuxue    |
|    3 | xiaohong | suzhou    |  3 | xiaohong   |    80 | english   |
|    3 | xiaohong | suzhou    |  4 | xiaohong   |    80 | physics   |
|    3 | xiaohong | suzhou    |  5 | xiaohong   |    80 | astronaut |
|    4 | xiaohei  | changchun | 14 | xiaohei    |    80 | shuxue    |
|    4 | xiaohei  | changchun | 15 | xiaohei    |    80 | physics   |
| NULL | NULL     | NULL      | 10 | 1.2xiaobai |    80 | english   |
| NULL | NULL     | NULL      | 11 | 2.2xiaobai |    80 | physics   |
| NULL | NULL     | NULL      | 12 | 3xiaobai   |    80 | shuxue    |
| NULL | NULL     | NULL      | 13 | 123xiaohei |    80 | astronaut |
| NULL | NULL     | NULL      | 16 | .12xiaohei |    80 | english   |
+------+----------+-----------+----+------------+-------+-----------+
16 rows in set (0.00 sec)