字符串转数字

正常的字符串数字直接转

mysql> select '123'+123;
+-----------+
| '123'+123 |
+-----------+
|       246 |
+-----------+
1 row in set (0.00 sec)

字符串中含有字母

字母在开头

mysql> select 'aa123'+123;
+-------------+
| 'aa123'+123 |
+-------------+
|         123 |
+-------------+
1 row in set, 1 warning (0.00 sec)

字母在数字中间

mysql> select '123aa12'+100;
+---------------+
| '123aa12'+100 |
+---------------+
|           223 |
+---------------+
1 row in set, 1 warning (0.00 sec)

字母在末尾

select '123bb'+100;
+-------------+
| '123bb'+100 |
+-------------+
|         223 |
+-------------+
1 row in set, 1 warning (0.00 sec)

总结

数字英文混杂的字符串进行计算，只能识别到字符串中开头的数字，一旦遇到不是数字的类型就舍弃之后所有的部分。

 select "aa" = 0 -------------true;

select "1aa"=0--------------false;

去除小数点后多余的0

select  0+cast(字段  as 字符类型)；

mysql> select '3.330000000'+0;
+-----------------+
| '3.330000000'+0 |
+-----------------+
|            3.33 |
+-----------------+
1 row in set (0.00 sec)