建立学生表
create table gh_student(
stuno varchar(3) primary key,
stuname varchar(4),
stuclass varchar(3)
)
插入数据
insert into gh_student values(‘001′,’gh’,’101′)
insert into gh_student values(‘002′,’lxg’,’102′)
insert into gh_student values(‘003′,’hs’,’103′)
建立成绩表
create table gh_grade(
grade_stuno varchar(3) ,
grade_lessonno varchar(3) ,
grade varchar(3)
)
插入数据
insert into gh_grade values(‘001′,’yw’,’98’)
GO
insert into gh_grade values(‘001′,’yy’,’99’)
GO
insert into gh_grade values(‘001′,’sx’,’94’)
GO
insert into gh_grade values(‘002′,’yw’,’93’)
insert into gh_grade values(‘002′,’yy’,’95’)
insert into gh_grade values(‘002′,’sx’,’96’)
在成绩表上建立外键约束 这里是级联更新
alter table gh_grade add constraint FK_StudentNo foreign key (grade_stuno) references gh_student (stuno) ON UPDATE CASCADE
此时执行select * from gh_grade
原始结果如下
grade_stuno grade_lessonno grade
001 yw 98
001 yy 99
001 sx 94
002 yw 93
002 yy 95
002 sx 96
注意此时演示级联更新
执行sql：update gh_student set stuno=’008′ where stuname=’gh’
再次执行查询语句：select * from grade
结果如下
grade_stuno grade_lessonno grade
008 yw 98
008 yy 99
008 sx 94
002 yw 93
002 yy 95
002 sx 96
接下来演示级联删除
建立级联删除
alter table gh_grade DROP constraint FK_StudentNo
alter table gh_grade add constraint FK_StudentNo foreign key (grade_stuno) references gh_student (stuno) ON DELETE CASCADE
然后执行sql：DELETE FROM gh_student WHERE stuno=’001′
再次执行查询：select * from grade
结果如下
grade_stuno grade_lessonno grade
002 yw 93
002 yy 95
002 sx 96
总结：所谓的级联更新，就是更新主键表（gh_student ）的同时，外键表(grade)同时更新。
所谓的级联删除，就是删除主键表（gh_student ）的同时，外键表(grade)同时删除。
这里还要注意：当学生表（gh_student ）中没有(‘001′,’gh’,’101′)这条记录时，成绩表(grade)中这三条插入语句是插不进去的，会报冲突，成绩表会自动判断没有001这个学生。
insert into gh_grade values(‘001′,’yw’,’98’)
GO
insert into gh_grade values(‘001′,’yy’,’99’)
GO
insert into gh_grade values(‘001′,’sx’,’94’)
GO
