MySQL中NOT IN语句对NULL值的处理

与使用in时不同:

在使用in 时:

SELECT COUNT(name) FROM CVE WHERE name NOT IN ('CVE-1999-0001', 'CVE-1999-0002');

SELECT COUNT(name) FROM CVE WHERE name NOT IN ('CVE-1999-0001', 'CVE-1999-0002',NULL);

二者结果相同;

在使用not in时:

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)