constraint foreign_key_name foreign key
(cloumn_name)
references database_name
(cloumn_name)
on update[on delete]
[no action]
[set null ]
[set default
][cascade]
no action ， set null ， set default ，cascade
no action 表示 不做任何操作，
set null 表示在外鍵表中將相應字段設定為null
set default 表示設定為預設值(restrict)
cascade 表示級聯操作，就是說，如果主鍵表中被參考字段更新，外來鍵表中也更新，主鍵表中的記錄被刪除，外來鍵表中改行也相應刪除
select * from springcloud.payment;
insert into payment
(serial)
values
('serial001');
# 方法一 外來鍵測試刪除
create table sys_user
(id bigint(20
)auto_increment primary key,
username varchar(50
),password varchar(50
));select * from sys_user;
select * from roles;
create table roles
(id int primary key not null,
role varchar(5
) not null);
insert into roles
(id,role)
value(1
,'admin');
insert into roles
(id,role)
values(2
,'user'),
(3,'view');
alter table sys_user add roleid int after password;
alter table sys_user add constraint roleid foreign key
(roleid) references roles
(id)
;update sys_user set roleid=
4 where id =1;
delete from sys_user where roleid =1;
delete from roles where roles.id=1;
insert into sys_user
(id,username,password,roleid)
value(1
,'admin'
,'000000',1
);# 方法二 級聯測試主表刪除同時附表刪除該記錄
select * from employee;
create table employee
(id bigint(20
)auto_increment primary key,
empname varchar(50
));insert into employee values(1
,'張三軟體開發部',1
);insert into employee values(2
,'李四軟體開發部',2
);insert into employee values(3
,'王五應用維護部',3
);select * from salary;
# 員工是主表，其他副表有員工的關聯屬性外來鍵，新增級聯效應刪除員工其他表一同刪除；
# 員工是主表，只有刪除了其他關聯了的屬性才可以刪除主表資訊；
create table salary
(id bigint(20
)auto_increment primary key,
number bigint(20
));insert into salary values(1
,5000);
insert into salary values(2
,6000);
insert into salary values(3
,7000);
alter table employee change column stuname empname varchar(30
); alter table employee change column salaryid salaryid bigint;# 型別修改一致
alter table employee add salaryid int after empname;#新增外來鍵字段
alter table employee add constraint employee_salaryid foreign key
(salaryid) references salary
(id) on delete cascade;#新增外來鍵約束並進行級聯關係
delete from employee where id =1;
delete from salary where id =
1;# 沒有這個人也就沒有相應工資表,工資表外來鍵員工id
# 方法三 級聯測試外來鍵直接建立約束foreign key
select * from student;
create table student
(id bigint(20
)auto_increment primary key,
stuname varchar(50
),resultid bigint(11
),constraint stu_resultid foreign key
(resultid) references result
(id));
select s.id,s.stuname,r.number from student s natural join result r where s.id = r.id;
insert into student values(1
,'張三',1
);insert into student values(2
,'李四',2
);insert into student values(3
,'王五',3
);select * from result;
create table result
(id bigint(20
)auto_increment primary key,
number int(5
));insert into result values(1
,70);
insert into result values(2
,80);
insert into result values(3
,90);
delete from student where id =1;
# 方法四 外來鍵約束foreign key
select * from course2;
create table course2
(id bigint(20
)auto_increment primary key,
coursename varchar(50
),studentid bigint,
constraint stuid foreign key
(studentid) references student2
(id));
delete from course2 where id=1;
insert into course2 values(1
,'語文',1
);insert into course2 values(2
,'數學',2
);insert into course2 values(3
,'化學',3
);select * from student2;
create table student2
(id bigint(20
)auto_increment primary key,
stuname varchar(50
));insert into student2 values(1
,'張三');
insert into student2 values(2
,'李四');
insert into student2 values(3
,'王五');
delete from student2 where id =
1;
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