我已经阅读了有关mysql的文档 IN 以及 NULL . 看这里：包含any、in或some的子查询，包含all的子查询
我做了一些实验。

创建表（MySQL5.6）

CREATE TABLE `test` (
  `uid` bigint(20) NOT NULL,
  PRIMARY KEY (`uid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO  test(uid) values (1),(2),(3),(4),(5);

声明1.1：

select 2 union all (select null);
+------+
|      |
+------+
|    2 |
| NULL |
+------+

声明1.2：

select uid from `test` where uid not in (select 2 union all (select null));  
+------+
|      |
+------+
|      |
+------+

声明1.3：

select uid from `test` where 1 not in (select 2 union all (select null));  
+------+
|      |
+------+
|      |
+------+

到目前为止，一切都按计划进行。但是现在我们修改了这个语句，混乱的事情发生了。

声明2.1：

select 2 union all (select null from test);
+------+
|      |
+------+
|   2  |
+------+
| NULL |
+------+
| NULL |
+------+
| NULL |
+------+
| NULL |
+------+
| NULL |
+------+

声明2.2：

select uid from `test` where uid not in (select 2 union all (select null from test));
+------+
|      |
+------+
|  3   |
+------+
|  4   |
+------+
|  5   |
+------+

声明2.3：

select uid from `test` where 1 not in (select 2 union all (select null from test)); 
+------+
|      |
+------+
|      |
+------+

声明2.4：

select uid from `test` where uid not in (select 3 union all (select null from test));
+------+
|      |
+------+
|  2   |
+------+
|  4   |
+------+
|  5   |
+------+

声明2.5：

select uid from `test` where 1 not in (select 3 union all (select null from test)); 
+------+
|      |
+------+
|      |
+------+

有人能解释2.2，2.4吗？当我们使用表达式或常量作为操作数时，结果是不一致的？为什么 1 没有出现在2.2和2.4的结果中？似乎2.3和2.5应该是正确的，正如mysql的文档所描述的那样。