我想删除父亲中的主键。所以我在child中设置外键来删除级联。为什么这不起作用？我需要先删除外键吗？如果是这样，那么在级联语句上删除的目的是什么？

create database table_test;

use table_test;

create table father(
cod int, 
primary key (cod)
);

create table child(
cod int,
cod_father int,
primary key(cod),
constraint fk_cod_father foreign key (cod_father) references father(cod)
on delete cascade
);

alter table father
drop primary key;