为什么 "where not in" 查询有 0 个结果

Question

mysql> show columns in m like 'fld';
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| fld   | varchar(45) | YES  | MUL | NULL    |       |
+-------+-------------+------+-----+---------+-------+
1 row in set (0.02 sec)

mysql> show columns in i like 'fld';
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| fld   | varchar(45) | YES  | MUL | NULL    |       |
+-------+-------------+------+-----+---------+-------+
1 row in set (0.02 sec)

mysql> select count(distinct fld) from i;
+---------------------+
| count(distinct fld) |
+---------------------+
|               27988 |
+---------------------+
1 row in set (0.03 sec)

mysql> select count(distinct fld) from m;
+---------------------+
| count(distinct fld) |
+---------------------+
|               72558 |
+---------------------+
1 row in set (0.07 sec)

根据我对相关表格的了解，上述结果似乎是合理的。

mysql> select count(*) from m where fld not in (select fld from i);
+----------+
| count(*) |
+----------+
|        0 |
+----------+
1 row in set (0.11 sec)

最后那个结果好像不太合理。 There must be 行在 m 中 fld 不在 i 中！有人可以解释为什么我得到 0 作为结果吗？

为了完整性（因为我怀疑它可能是相关的），我还会粘贴这个结果：

mysql> select count(*) from m where fld is null;
+----------+
| count(*) |
+----------+
|        0 |
+----------+
1 row in set (0.00 sec)

编辑：为了回复评论，我也在编辑以下信息，以防有人回答我的问题：

select count(*) from m join i using (fld) 产生 9350； left join, 73087; right join, 28872.
select count(*) from i where fld is null 产量 810.

Answer 1

我现在明白了。 The documentation 读取：

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.

因此，只要在派生的 table（这里有）并且在派生的 table 中找不到 fld（这是我正在测试的内容）。

可能有比这更好的解决方法，但我使用的是 (select fld from i where fld is not null)。

为什么 "where not in" 查询有 0 个结果

Why 0 results on "where not in" query

mysql

where-in

mysql-5.6