null和null不相等吗？

本人有一个查询，
where m1.vpassword = m2.vpassword 光有这个条件不能把vpassword=null的情况查询处理
or m1.vpassword is null and m2.vpassword is null 必须加上这一行，不是很变态吗？
问一下这是怎么回事？

万能的MYSQL免费手册。

引用

<=>
NULL-safe equal. This operator performs an equality comparison like the = operator, but returns 1 rather than NULL if both operands are NULL, and 0 rather than NULL if one operand is NULL.
mysql> SELECT 1 <=> 1, NULL <=> NULL, 1 <=> NULL;
-> 1, 1, 0
mysql> SELECT 1 = 1, NULL = NULL, 1 = NULL;
-> 1, NULL, NULL