drop table dept cascade constraints

同事问我  drop table dept cascade constraints; 这个关键字  cascade constraints 的作用.

应该是在父子表中存在外键关联的时候 drop 父表的时候级联drop 子表的外键.

简单记录了测试过程.

SQL>
SQL> create table dept (dept_id number , dept_name varchar2(20));
 
Table created
 
SQL> alter table dept add CONSTRAINTS pk_dept primary key (dept_id);
 
Table altered
 
SQL> create table emp (emp_id number , dept_id number , emp_name varchar2(20));
 
Table created
 
SQL> alter table emp add constraints pk_emp primary key (emp_id);
 
Table altered
 
SQL> alter table emp add constraints fk_emp foreign key (dept_id ) references dept(dept_id);
 
Table altered

SQL> insert into dept (dept_id,dept_name) values (1,'dept1');
 
1 row inserted
 
SQL> insert into dept (dept_id,dept_name) values (2,'dept2');
 
1 row inserted
 
SQL> commit;
 
Commit complete
 
SQL> insert into emp(emp_id,dept_id,emp_name) values(1,1,'emp1');
 
1 row inserted
 
SQL> insert into emp(emp_id,dept_id,emp_name) values(2,2,'emp1');
 
1 row inserted
 
SQL> commit;
 
Commit complete
 
SQL>
SQL> drop table dept;
 
drop table dept
 
ORA-02449: 表中的唯一/主键被外部关键字引用
 
SQL>
SQL> drop table dept cascade constraints;
 
Table dropped
 
SQL> SELECT constraint_name FROM User_Constraints;
 
CONSTRAINT_NAME
------------------------------
PK_EMP
 
SQL>