SQL外键

外键用来与某些数据表建立联系，保证数据一致性

 

create table users
(
id int identity not null,
names varchar(50) not null,
n_id int not null,
primary key(n_id),
)

create table books
(
id int identity not null,
b_name varchar(50) not null,
b_id int not null,
primary key(id),
foreign key(b_id) references users(n_id)
)
select a.names as '作者',b.b_name as '书名',b.b_id as '关联ID' 
from users a left join books b on a.n_id=b.b_id

insert into users(names,n_id) values('Jim',78)

insert into books(b_name,b_id) values('C#高级编程',78)
上面SQL语句在books表中建立外键b_id,在执行update 或者insert语句时，如果books 表中的b_id 在user表中n_id不对应时就会执行失败