为什么 select NOT IN returns 0 row 而实际上有很多结果

Question

如果我运行:

select ANY_VALUE(id)
from my_table
group by title

它将 return 5563 行，这些行都是具有不同 'title'.

的行的 ID

Table my_table 共有 6497 行。（如果我运行: Select * from my_table 我有 6497 行）

现在我希望这个查询能给我 my_table 中的其余 id（934 行）的列表：

Select * from my_table where id NOT IN (
select ANY_VALUE(id)
from my_table
group by title)

但它给了我 0 行。

我也试过：

  Select * from my_table where id NOT IN (
    select ANY_VALUE(id)
    from my_table
    group by title) AND id IS NOT NULL

或

Select * from my_table  where not exists (
select ANY_VALUE(id) AS value
from my_table
group by title) and id is not NULL;

全部return 0行。我做错了什么？

Answer 1

我会说这是一个错误。

重现：

create table foo(id int auto_increment primary key, a int);
insert into foo(a) values(1), (1), (2);

当你执行 explain 和 show warnings 时（在 MySQL <= 5.6 中你必须执行 explain extended）你可以看到 MySQL在优化器完成他的工作后实际执行：

mysql > explain select * from foo where id not in (select id from foo group by a);
+----+--------------------+-------+------------+-----------------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type        | table | partitions | type            | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+--------------------+-------+------------+-----------------+---------------+---------+---------+------+------+----------+-------------+
|  1 | PRIMARY            | foo   | NULL       | ALL             | NULL          | NULL    | NULL    | NULL |    3 |   100.00 | Using where |
|  2 | DEPENDENT SUBQUERY | foo   | NULL       | unique_subquery | PRIMARY       | PRIMARY | 4       | func |    1 |   100.00 | NULL        |
+----+--------------------+-------+------------+-----------------+---------------+---------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

mysql > show warnings;
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message                                                                                                                                                                                                                                               |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note  | 1003 | /* select#1 */ select `playground`.`foo`.`id` AS `id`,`playground`.`foo`.`a` AS `a` from `playground`.`foo` where (not(<in_optimizer>(`playground`.`foo`.`id`,<exists>(<primary_index_lookup>(<cache>(`playground`.`foo`.`id`) in foo on PRIMARY))))) |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

如您所见，这是一个完全不同的查询。没有更多的分组。要获得您期望的结果，您必须像这样再次嵌套查询：

select * from foo where id not in (
    select id from (
        select any_value(id) as id from foo group by a
    ) sq
);

为什么 select NOT IN returns 0 row 而实际上有很多结果

Why select NOT IN returns 0 row while there are actually many results

mysql

notin