postgresql: ‘not in’ subquery

在使用sql语句进行查询时突然发现一个语句不能返回合适结果：

select count(*) from weibo.poi where poiid not in (select poiid from weibo.url) and poiid not in (select poiid from weibo.url_bad);

正常应该有结果，但此语句返回空集。
搜索后发现后面的not in语句中存在null值，导致返回空集。因此应该这样修改：

select count(*) from weibo.poi where poiid not in (select poiid from weibo.url) and poiid not in (select poiid from weibo.url_bad where poiid is not null);