alter table emp add constraint emp_deptno_fk foreign key(deptno) references dept (deptno) on delete cascade;
如代码块,contraint后面的字符串emp_deptno_fk是干什么的
alter table emp add constraint emp_deptno_fk foreign key(deptno) references dept (deptno) on delete cascade;
如代码块,contraint后面的字符串emp_deptno_fk是干什么的