自动维护约束完整一致性。

on delete子句可以保证数据的一致性，如下例中，删除t1表记录时，相应的t2表记录将被删除：

create table a_t1(id int primary key,name varchar(20));
create table a_t2(id int primary key references a_t1(id) on delete cascade,qty number)

对于on delete子句的解释如下：

ON DELETE Clause

The ON DELETE clause lets you determine how Oracle automatically maintains referential integrity if you remove a referenced primary or unique key value. If you omit this clause, then Oracle does not allow you to delete referenced key values in the parent table that have dependent rows in the child table.

• Specify CASCADE if you want Oracle to remove dependent foreign key values.
• Specify SET NULL if you want Oracle to convert dependent foreign key values to NULL.

CREATE TABLE dept_20
   (employee_id     NUMBER(4) PRIMARY KEY,
    last_name       VARCHAR2(10),
    job_id          VARCHAR2(9),
    manager_id      NUMBER(4) CONSTRAINT fk_mgr
                    REFERENCES employees ON DELETE SET NULL,
    hire_date       DATE,
    salary          NUMBER(7,2),
    commission_pct  NUMBER(7,2),
    department_id   NUMBER(2)   CONSTRAINT fk_deptno
                    REFERENCES departments(department_id)
                    ON DELETE CASCADE );