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);
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