在MySQL中将字符串与“大于”和“小于”比较是安全的吗？ [英] Is it safe to compare strings with 'greater than' and 'less than' in MySQL?

问题描述

MySQL（5.1.41-3ubuntu12.10-log）似乎可以使用>（大于）和< （小于）：

MySQL (5.1.41-3ubuntu12.10-log) seems to give predictable results on string comparison using > (greater than) and < (less than):

select "a" > "a", "a" > "b", "b" > "a", "ab" > "aa", "ab" > "aabbbb";
+-----------+-----------+-----------+-------------+-----------------+
| "a" > "a" | "a" > "b" | "b" > "a" | "ab" > "aa" | "ab" > "aabbbb" |
+-----------+-----------+-----------+-------------+-----------------+
|         0 |         0 |         1 |           1 |               1 | 
+-----------+-----------+-----------+-------------+-----------------+

并且似乎利用了键：

explain select productcode from products where productcode < 'no'; 
+----+-------------+----------+-------+-----------------+------+---------+------+------+--------------------------+
| id | select_type | table    | type  | possible_keys   | key  | key_len | ref  | rows | Extra                    |
+----+-------------+----------+-------+-----------------+------+---------+------+------+--------------------------+
|  1 | SIMPLE      | products | range | productcode,ppp | ppp  | 34      | NULL |  432 | Using where; Using index |
+----+-------------+----------+-------+-----------------+------+---------+------+------+--------------------------+

这不会

推荐答案

我认为有一些问题，你可以有关详细信息，请查看文档：

I think there are some gotchas, you can have a look at documentation here for some details :

http://dev.mysql.com/doc/refman/5.0/en/string-comparison-functions.html

如果你的字段也有空值，你还应该看看一个null安全的比较运算符：
http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html#operator_equal-to

If your fields have null values too, you should also take a look at null-safe comparision operator: http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html#operator_equal-to

例如：

mysql> select "a" > "a ", "A" > "a" , "aB"  > "ab" , "a" >= NULL , "a" <=> NULL ;
+------------+-----------+--------------+-------------+--------------+
| "a" > "a " | "A" > "a" | "aB"  > "ab" | "a" >= NULL | "a" <=> NULL |
+------------+-----------+--------------+-------------+--------------+
|          0 |         0 |            0 |        NULL |            0 |
+------------+-----------+--------------+-------------+--------------+

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