mysql> select count(name) from cve where name not in ('cve-1999-0001', 'cve-1999-0002');
+-------------+
| count(name) |
+-------------+
| 17629 |
+-------------+
1 row in set (0.02 sec)
mysql> select count(name) from cve where name not in ('cve-1999-0001', 'cve-1999-0002', null);
+-------------+
| count(name) |
+-------------+
| 0 |
+-------------+
1 row in set (0.01 sec)
当在子查询中出现null的时候，结果就一定是0了。查了一下手册，确实有这样的说法。所以最后实际采用了这样的查询：
select count(distinct name)
from cve
where name not in (select cveid from cve_sig where cveid is not null)
顺便提一下mysql中正则表达式匹配的简单使用：
select count(alarmid)
from alarm
where (cve not rlike '^cve-[0-9]{4}-[0-9]{4}$' or cve is null)
当然，rlike也可以写作regexp，我个人倾向于使用rlike，因为拼写接近like，可以见名知义。
，