1.創(chuàng)建測試表：

drop table student;

drop table class;

drop table student_class;

Create table student(student_id integer not null,student_name varchar(200), CONSTRAINT P_KEY_1 primary key (student_id)) in luzl_32k_tb index in luzl_32k_tb ;

Create table class(class_id integer not null,class_name varchar(200), CONSTRAINT P_KEY_2 primary key (class_id)) in luzl_32k_tb index in luzl_32k_tb ;

Create table student_class(student_class_id integer,student_id integer,class_id integer) in luzl_32k_tb index in luzl_32k_tb;

alter table student_class add constraint if_class foreign key(class_id) references class(class_id) ON DELETE cascade ON UPDATE RESTRICT;

alter table student_class add constraint if_student foreign key(student_id) references student(student_id) ON DELETE cascade ON UPDATE RESTRICT;

Insert into student(student_id,student_name) values(1,'luzl');

Insert into class(class_id,class_name) values(1,'db2');

Insert into student_class(student_class_id,student_id,class_id) values(1,1,1);

2.On Delete 的選項(xiàng)有

Restrict/no action/cascade/set null.其中cascade選項(xiàng)指定的話，如果刪除父記錄，依賴于他的子記錄也會自動刪除.相當(dāng)于級聯(lián)刪除.如果指定no action和cascade都會報錯，因?yàn)檫€有子記錄所以無法刪除該記錄.set nul允許刪除父記錄并且l會將子表中與父表關(guān)聯(lián)的字段設(shè)置為null.

3.On Update 只有兩個選項(xiàng) no action/restrict.它們在更新和刪除時并沒有區(qū)別:如果與子表關(guān)聯(lián)不允許刪除.

4.另外還需要注意一點(diǎn)，父表中的字段必須是主鍵，才能做為子表的外鍵。