mysql在比较时，首先会进行类型转型，由于是自动的，所以很难被发现，比如

select 1='1sdjfksdjfksdf';

select 1+'2';

mysql在操作数时默认会发生类型转换，字符串与数字操作时，字符串会转为数字。
比如

select 1='1aaaa'; //结果,1 
select 11='11bbb'; //结果,1

那么这一条数据会被查询出来，因为11michael会自动转换为11

官网的例子：

For comparisons of a string column with a number, MySQL cannot use an
index on the column to look up the value quickly. If str_col is an
indexed string column, the index cannot be used when performing the
lookup in the following statement:
SELECT * FROM tbl_name WHERE str_col=1; The reason for this is that
there are many different strings that may convert to the value 1, such
as ‘1’, ’ 1’, or ‘1a’.

参考

• IT虾米网