知識點的梳理：

on delete cascade：當主表資料刪除時，對應的子表資料同時刪除；

on delete set null：當主表資料刪除時，對應的子表資料設定為null；

問題1：刪除父表資料前需要先刪除所有子表的對應資料

由於子表的所有資料都要和父表的資料對應，所以在刪除父表資料時需要先將子表中對應的資料刪除乾淨，否則將無法刪除

示例1：刪除member表中編號為“1”的資料（mid=1），此時沒有刪除子表（advice）資料

DELETE FROM member WHERE mid=1 ;

member中的mid資料與advice有關聯關係，所以只能先刪除advice

DELETE FROM advice WHERE mid=1 ;
DELETE FROM member WHERE mid=1 ;
COMMIT ;

這樣的刪除較為複雜，可以通過級聯操作來簡化操作！

DROP TABLE advice PURGE ;
DROP TABLE member PURGE ;
CREATE TABLE member (
    mid       NUMBER    ,
    name      VARCHAR2(200)   NOT NULL ,
    CONSTRAINT pk_mid PRIMARY KEY (mid) 
) ;
CREATE TABLE advice (
    adid      NUMBER ,
    content   CLOB            NOT NULL ,
    mid       NUMBER ,
    CONSTRAINT pk_adid PRIMARY KEY (adid) ,
    CONSTRAINT fk_mid FOREIGN KEY(mid) REFERENCES member(mid) ON DELETE CASCADE
) ;
INSERT INTO member (mid,name) VALUES (1,'李興華') ; 
INSERT INTO member (mid,name) VALUES (2,'董鳴楠') ;
INSERT INTO advice (adid,content,mid) VALUES (1,'應該提倡內部溝通機制，設定總裁郵箱',1) ;
INSERT INTO advice (adid,content,mid) VALUES (3,'要多開展員工培訓活動，讓員工更加有歸屬感',2) ;
COMMIT ;
 

此時刪除member表中編號為1的成員資訊，可以觸發級聯操作，完成刪除動作；

DELETE FROM member WHERE mid=1 ;

DROP TABLE advice PURGE ;
DROP TABLE member PURGE ;
CREATE TABLE member (
    mid       NUMBER    ,
    name      VARCHAR2(200)   NOT NULL ,
    CONSTRAINT pk_mid PRIMARY KEY (mid) 
) ;
CREATE TABLE advice (
    adid      NUMBER ,
    content   CLOB            NOT NULL ,
    mid       NUMBER ,
    CONSTRAINT pk_adid PRIMARY KEY (adid) ,
    CONSTRAINT fk_mid FOREIGN KEY(mid) REFERENCES member(mid) ON DELETE SET NULL
) ;
INSERT INTO member (mid,name) VALUES (1,'hey') ; 
INSERT INTO member (mid,name) VALUES (2,'yan') ;
INSERT INTO advice (adid,content,mid) VALUES (1,'應該提倡內部溝通機制，設定總裁郵箱',1) ;
INSERT INTO advice (adid,content,mid) VALUES (3,'要多開展員工培訓活動，讓員工更加有歸屬感',2) ;
COMMIT ;

DELETE FROM member WHERE mid=1 ;

問題2：刪除父表時需要先將子表刪除

如果想刪除父表，只能先刪除子表，在刪除父表

DROP TABLE advice PURGE ;
DROP TABLE member PURGE ;

DROP TABLE member CASCADE CONSTRAINT ;