AND字段NOT IN(NULL)返回一个空集

此查询为什么返回一个空集:

Why does this query returns an empty set:

select p.iuser_id,p.produit_id from portefeuille p
WHERE produit_id=48
AND p.iuser_id NOT IN (NULL);

而这个:

select p.iuser_id,p.produit_id from portefeuille p
WHERE produit_id=48
LIMIT 5

返回结果，例如

72968, 48
106967, 48
7381, 48
81678, 48
194250, 48

，并且这些值都不为NULL或应等于NULL. (为了简洁起见，我添加了限制5，并且我正在使用MySql 5.1)

and none of those values are NULL or should be equal to NULL. (I added limit 5 for consiseness, and I am using MySql 5.1)

edit:在这里，我将问题缩小为NOT IN(NULL).原始查询有一个子查询，其中某些行包含NULL，如下所示:

edit: Here I narrowed the problem down to NOT IN(NULL). The original query had a subquery with some of the rows containing NULL as such:

WHERE user_id NOT IN( select user_id from mailsubscriptions )

和一些user_id为NULL，但是集合中只有一个NULL会污染整个查询.

and some user_id were NULL, but only one NULL in the set contaminates the whole query.