mysql连接查询（左连接，右连接，内连接）

mysql> select * from name_address;
+----------+------+----+
| address | name | id |
+----------+------+----+
| 西北一路 | 张三 | 1 |
| 西北二路 | 李四 | 2 |
| 西北三路 | 王五 | 3 |
+----------+------+----+
3 rows in set

mysql> select * from name_age;
+-----+--------+----+
| age | name  | id |
+-----+--------+----+
| 18 | 张三  | 1 |
| 20 | 王五  | 2 |
| 21 | 路人甲 | 3 |
+-----+--------+----+
3 rows in set

mysql> SELECT a.`name`,a.age,b.address FROM name_age a INNER JOIN name_address b WHERE（on） a.`name`=b.`name`;
+------+-----+----------+
| name | age | address |
+------+-----+----------+
| 张三 | 18 | 西北一路 |
| 王五 | 20 | 西北三路 |
+------+-----+----------+
2 rows in set

mysql> SELECT a.`name`,a.age,b.address FROM name_age a left JOIN name_address b on
 a.`name`=b.`name`;
+--------+-----+----------+
| name  | age | address |
+--------+-----+----------+
| 张三  | 18 | 西北一路 |
| 王五  | 20 | 西北三路 |
| 路人甲 | 21 | NULL   |
+--------+-----+----------+
3 rows in set

mysql> SELECT b.`name`,a.age,b.address FROM name_age a right JOIN name_address b on a.`name`=b.`name`;
+------+------+----------+
| name | age | address |
+------+------+----------+
| 张三 | 18  | 西北一路 |
| 王五 | 20  | 西北三路 |
| 李四 | NULL | 西北二路 |
+------+------+----------+
3 rows in set