【MySQL】解决MySQL删除外键(foreign key)时报错问题

文章目录

【MySQL】解决MySQL删除外键(foreign key)时报错问题
- 一、现象
- 二、分析
- 三、问题解决
- 四、拓展

一、现象

(1) 查看主表class_info和从表student。

MariaDB [class_info]> desc class_info;
+------------+-------------+------+-----+---------+-------+
| Field      | Type        | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| class_id   | int(2)      | NO   | PRI | 0       |       |
| class_name | varchar(10) | YES  |     | NULL    |       |
+------------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)MariaDB [class_info]> desc student;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| stu_id   | int(2)      | YES  |     | NULL    |       |
| name     | varchar(10) | YES  |     | NULL    |       |
| class_id | int(2)      | YES  |     | NULL    |       |
| score    | float(4,2)  | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

(2) 在从表student上创建外键

MariaDB [class_info]> alter table student add foreign key (class_id) references class_info(class_id);
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0MariaDB [class_info]> desc student;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| stu_id   | int(2)      | YES  |     | NULL    |       |
| name     | varchar(10) | YES  |     | NULL    |       |
| class_id | int(2)      | YES  | MUL | NULL    |       |
| score    | float(4,2)  | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

(3) 在删除外键时抛出这样的错误

MariaDB [class_info]> alter table student drop foreign key class_id;
ERROR 1025 (HY000): Error on rename of './class_info/student' to './class_info/#sql2-1a32-13e' (errno: 152)

二、分析

MySQL不能删除外键，报错 Error on rename of './class_info/student' to './class_info/#sql2-1a32-13e' (errno: 152)

其实这是MySQL的一个不友好的地方而已，主要有以下几个原因：

(1) MySQL在建外键后,会自动建一个同名的索引。

(2) 在删除外键时，如果这个同名索引没有被删除，则MySQL认为外键仍然是存在的，在show index from [表名]命令里仍旧会显示外键。

那么如何进行外键的删除呢？

三、问题解决

要删除外键可以分三步：

第一步： 查看外键约束：show create table [表名]；

MariaDB [class_info]> show create table student\G
*************************** 1. row ***************************Table: student
Create Table: CREATE TABLE `student` (`stu_id` int(2) DEFAULT NULL,`name` varchar(10) DEFAULT NULL,`class_id` int(2) DEFAULT NULL,`score` float(4,2) DEFAULT NULL,KEY `class_id` (`class_id`),CONSTRAINT `student_ibfk_1` FOREIGN KEY (`class_id`) REFERENCES `class_info` (`class_id`),
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

第二步： 删除同名索引：alter table [表名] drop foreign key [同名索引];

这里的同名索引是 CONSTRAINT 后面的 student_ibfk_1。可以看到在删除同名索引之后，外键仍然是存在的。

MariaDB [class_info]> alter table student drop foreign key student_ibfk_1;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0MariaDB [class_info]> show create table student\G
*************************** 1. row ***************************Table: student
Create Table: CREATE TABLE `student` (`stu_id` int(2) DEFAULT NULL,`name` varchar(10) DEFAULT NULL,`class_id` int(2) DEFAULT NULL,`score` float(4,2) DEFAULT NULL,KEY `class_id` (`class_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)MariaDB [class_info]> show index from student\G
*************************** 1. row ***************************Table: studentNon_unique: 1Key_name: class_idSeq_in_index: 1Column_name: class_idCollation: ACardinality: 0Sub_part: NULLPacked: NULLNull: YESIndex_type: BTREEComment:
Index_comment:
1 row in set (0.00 sec)MariaDB [class_info]> desc student;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| stu_id   | int(2)      | YES  |     | NULL    |       |
| name     | varchar(10) | YES  |     | NULL    |       |
| class_id | int(2)      | YES  | MUL | NULL    |       |
| score    | float(4,2)  | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+

第三步： 删除索引：alter table [表名] drop index [字段名]; 可以看到此时外键约束被删彻底删除。

MariaDB [class_info]> alter table student drop index class_id;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0MariaDB [class_info]> show index from student\G
Empty set (0.00 sec)MariaDB [class_info]> desc student;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| stu_id   | int(2)      | YES  |     | NULL    |       |
| name     | varchar(10) | YES  |     | NULL    |       |
| class_id | int(2)      | YES  |     | NULL    |       |
| score    | float(4,2)  | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

四、拓展

MySQL中如果表和表之间建立的外键约束，则无法删除表及修改表结构。

解决方法： 在MySQL中取消外键约束: set foreign_key_checks = 0; 然后将原有表数据导出到sql语句，重新创建此表后，再把数据使用sql导入，最后设置外键约束: set foreign_key_checks = 1;