sql - MySQL SELECT x FROM a WHERE NOT IN (SELECT x FROM b) - 意外结果

Question

我希望下面第三个查询的结果包含 id=732。它没有。这是为什么呢？

mysql> SELECT id FROM match ORDER BY id DESC LIMIT 5 ;
+------------+
|         id |
+------------+
|        732 | 
|        730 | 
|        655 | 
|        458 | 
|        456 | 
+------------+
5 rows in set (0.00 sec)

mysql> SELECT id FROM email ORDER BY id DESC LIMIT 5 ;
+------------+
|         id |
+------------+
|        731 | 
|        727 | 
|        725 | 
|        724 | 
|        723 | 
+------------+
5 rows in set (0.00 sec)

mysql> SELECT * FROM match WHERE id NOT IN ( SELECT id FROM email ) ;
Empty set (0.00 sec)

email.id 表中有 3 个 NULL 条目，match.id 中没有 NULL 条目。

完整的表/查询可以在 http://pastebin.ca/1462094 看到

Answer 1

来自 documentation :

To comply with the SQL standard, IN returns NULL not only if the expression on the left hand side is NULL, but also if no match is found in the list and one of the expressions in the list is NULL.

这正是你的情况。

IN 和 NOT IN 都返回 NULL，这不是 WHERE 子句可接受的条件。

按如下方式重写您的查询:

SELECT  *
FROM    match m
WHERE   NOT EXISTS
        (
        SELECT  1
        FROM    email e
        WHERE   e.id = m.id
        )