在mariadb的关键列中的Mul值

Question

我在mariadb中创建了dept表，其中dno为主键。然后我创建了emp表，其中eno作为主键，dno作为外键，如下所示：

create table dept(dno int primary key,dname varchar(20));
create table emp(eno int primary key,ename varchar(20),dno int,foreign key(dno) references dept(dno));

然后我描述了两个表

DESC emp;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| eno   | int(11)     | NO   | PRI | NULL    |       |
| ename | varchar(20) | YES  |     | NULL    |       |
| dno   | int(11)     | YES  | MUL | NULL    |       |
+-------+-------------+------+-----+---------+-------+

desc dept;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| dno   | int(11)     | NO   | PRI | NULL    |       |
| dname | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

从emp表中删除外键dno后，它在emp表中显示了相同的内容

+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| eno   | int(11)     | NO   | PRI | NULL    |       |
| ename | varchar(20) | YES  |     | NULL    |       |
| dno   | int(11)     | YES  | MUL | NULL    |       |
+-------+-------------+------+-----+---------+-------+

如果删除外键约束

，如何删除mul值键列