mysql中读取一个字符串长度的示例：
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 length(name) from test where id=3;（查询id为3的name字符串长度）
+--------------+
| length(name) |
+--------------+
| 8 |
+--------------+
1 row in set (0.00 sec)
