mysql 获取字符串的长度

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 length(name) from test where id=;
+--------------+
| length(name) |
+--------------+
|             |
+--------------+
 row in set (0.00 sec)