創建外鍵時的選項：
1.創建測試表：
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 的選項有
Restrict/no action/cascade/set null.其中cascade選項指定的話，如果刪除父記錄，依賴於他的子記錄也會自動刪除.相當於級聯刪除.如果指定no action和cascade都會報錯，因為還有子記錄所以無法刪除該記錄.set nul允許刪除父記錄並且l會將子表中與父表關聯的字段設置為null.
3.On Update 只有兩個選項 no action/restrict.它們在更新和刪除時並沒有區別:如果與子表關聯不允許刪除.
4.另外還需要注意一點，父表中的字段必須是主鍵，才能做為子表的外鍵。
