刚刚才知道MySQL的varchar类型比较默认是忽略大小写的，还忽略最后的空格。C.5.5.1. Case Sensitivity in String Searches
mysql> SELECT 'a' = 'A';
-> 1
解决方案是使用BINARY操作符10.1.7.7. The BINARY Operator
mysql> SELECT BINARY 'a' = 'A';
-> 0
mysql> SELECT 'a' = 'a ';
-> 1
mysql> SELECT BINARY 'a' = 'a ';
-> 0
