问题描述
student table:
+---------------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------------+----------+------+-----+---------+-------+
| stu_id | int | NO | PRI | NULL | |
| stu_name | char(30) | YES | UNI | NULL | |
| stu_branch_id | int | NO | MUL | NULL | |
+---------------+----------+------+-----+---------+-------+
branch table:
+--------------------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------------+----------+------+-----+---------+-------+
| branch_id | int | NO | PRI | NULL | |
| branch_name | char(30) | YES | UNI | NULL | |
| branch_building_no | int | YES | | NULL | |
+--------------------+----------+------+-----+---------+-------+
MysqL> alter table student add constraint fk_student foreign key (stu_branch_id) references branch(branch_id);
Query OK,5 rows affected (2.69 sec)
Records: 5 Duplicates: 0 Warnings: 0
desc student;
+---------------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------------+----------+------+-----+---------+-------+
| stu_id | int | NO | PRI | NULL | |
| stu_name | char(30) | NO | | NULL | |
| stu_branch_id | int | NO | MUL | NULL | |
+---------------+----------+------+-----+---------+-------+
3 rows in set (0.00 sec)
MysqL> alter table student
-> drop foreign key fk_student;
Query OK,0 rows affected (0.21 sec)
Records: 0 Duplicates: 0 Warnings: 0
MysqL> desc student;
+---------------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------------+----------+------+-----+---------+-------+
| stu_id | int | NO | PRI | NULL | |
| stu_name | char(30) | YES | UNI | NULL | |
| stu_branch_id | int | NO | MUL | NULL | |
+---------------+----------+------+-----+---------+-------+
3 rows in set (0.01 sec)
MysqL> alter table student
-> add constraint fk_student foreign key (stu_id_branch) references branch(branch_id) on delete cascade;
ERROR 1061 (42000): Duplicate key name 'fk_student'
删除外键后，MUL 键还在。 现在，当我尝试添加另一个具有相同名称的外键时，出现错误。
解决方法
隐式索引创建导致的问题。创建外键 fk_student 时，索引 fk_student 也创建了。如果您需要回滚 FK 创建，您需要下一步：
alter table student drop foreign key fk_student,drop key fk_student;