为什么我的 `delete where id in` 语句会删除表中的所有记录？

Question

我正在使用 MySQL 版本 5.7.24。 我想删除回复表中具有相同 ex_id 和 ex_type 的记录：

CREATE TABLE `reply` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `content` varchar(1024) NOT NULL,
  `ex_id` bigint(20) DEFAULT '0',
  `ex_type` int(11) DEFAULT '0',
  PRIMARY KEY (`id`),
  KEY `idx_ex_id_type` (`ex_id`,`ex_type`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

有以下数据：

+----+-------------------+-------+---------+
| id | content           | ex_id | ex_type |
+----+-------------------+-------+---------+
|  1 | this is a content |     1 |       1 |
|  2 | this a test       |     2 |       1 |
|  3 | this a contet     |     1 |       1 |
|  4 | the 4th content   |     3 |       1 |
+----+-------------------+-------+---------+

记录 1 和 3 共享相同的 ex_id 和 ex_type，其中我想删除具有较小 id 的记录（记录 1），所以我编写了以下查询：

delete from reply where id in (
    select id from (
        select min(id) from reply group by ex_type and ex_id having count(1) > 1
    ) tmp
)
-- Query OK, 4 rows affected

这应该删除一条记录，但所有记录都被删除。

Actually there is a mistake in this SQL, the inner SQL select min(id) from reply group by ex_type and ex_id having count(1) > 1 return the result with only field: 'min(id)', outer sql select id from () tmp选择了一个不存在的字段 id 导致错误，但是 MySQL 仍然执行这个 sql 并删除所有记录。

我想知道为什么会这样。

Answer 1

您的查询在逻辑上是错误的。
这部分：

select min(id) from reply group by ex_type and ex_id having count(1) > 1

按ex_type and ex_id而不是ex_type, ex_id 。
最重要的是，它不会返回名为/别名为id的列。
以便：

select id from...

实际上是指表的id ，返回表的所有id ，结果就是所有的行都被删除了。
您可以在此处查看此行为。
我相信这是你想要做的：

delete from reply where id in 
(
    select id from (
        select min(id) id from reply group by ex_type, ex_id having count(*) > 1
    ) tmp
); 

Answer 2

...outer sql select id from () tmp a non exsit field id which lead to an error, but mysql execute this sql and delete all records.

我想知道为什么会这样。

此子查询不会自行运行：

select id from (
    select min(id) from reply group by ex_type and ex_id having count(1) > 1
) tmp
/* SQL Error (1054): Unknown column 'id' in 'field list' */

但是当它在子查询中运行时，根据 scope 解析规则，id 列解析为外部查询的 id 列，因为请求的列不存在于 FROM 子句中。 查询本质上是这样的：

delete from reply where id in (
    select reply.id from (
        select min(id) from reply group by ex_type and ex_id having count(1) > 1
    ) tmp
)
/* Affected rows: 4  Found rows: 0  Warnings: 0  Duration for 1 query: 0.031 sec. */

该条件对所有行都为真，因为 1 IN (1)、2 IN (2)、3 IN (3)... 都为真。 修复错字（ group by ex_type and ex_id ）不会解决问题，请将您的查询更改为：

delete from reply where id in (
    select tmp.id from (
        select min(id) as id from reply group by ex_type, ex_id having count(1) > 1
    ) tmp
)

Answer 3

有趣 - 这似乎是一个错误，因为 DELETE 在这种情况下应该失败！

无论如何，只需将别名附加到 min(id) （并在 GROUP BY 之前删除那个奇怪的“和”），一切都会好起来的......（虽然，我不会这样写这个查询）

DROP TABLE IF EXISTS reply;

CREATE TABLE `reply` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `content` varchar(1024) NOT NULL,
  `ex_id` bigint(20) DEFAULT '0',
  `ex_type` int(11) DEFAULT '0',
  PRIMARY KEY (`id`),
  KEY `idx_ex_id_type` (`ex_id`,`ex_type`)
);

INSERT INTO reply VALUES
(1,'this is a content',1,1),
(2,'this a test',2,1),
(3,'this a contet',1,1),
(4,'the 4th content',3,1);


delete from reply where id in 
(
    select id from (
        select min(id) id from reply group by ex_type, ex_id having count(1) > 1
    ) tmp
);

SELECT * FROM reply;
+----+-----------------+-------+---------+
| id | content         | ex_id | ex_type |
+----+-----------------+-------+---------+
|  2 | this a test     |     2 |       1 |
|  3 | this a contet   |     1 |       1 |
|  4 | the 4th content |     3 |       1 |
+----+-----------------+-------+---------+

FWIW，对于一个小数据集，我可能会这样写那个查询......

DELETE r 
  FROM reply r
  JOIN 
     ( SELECT MIN(id) id 
         FROM reply 
        GROUP 
           BY ex_type
            , ex_id 
       HAVING COUNT(0) > 1
     ) x
    ON x.id = r.id