MySQL中NOT IN语句对NULL值的处理

MySQL中NOT IN语句对NULL值的处理

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。