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一、在使用数据库保存信息产生数据冗杂现象
create table user_info(
id char(36) primary key,
user_name varchar(30) not null,
password varchar(30) not null,
real_name varchar(8),
mobile char(11),
address varchar(150)
);
insert into user_info (id,user_name,password,real_name,mobile,address)
values ('51b28fe1-4ebf-41ac-a17b-d5e276861fd0','fuliuqingfeng','123456','张三','18920120206','河南安阳');
insert into user_info (id,user_name,password,real_name,mobile,address)
values ('cc95772b-75a2-4702-bd99-4c3b0322d606','fuliuqingfeng','123456','李四','18617297545','北京海淀');
insert into user_info (id,user_name,password,real_name,mobile,address)
values ('c63028fd-cf8d-4dac-a278-b5cc8fd61e3c','fuliuqingfeng','123456','王五','17694976949','山西大同');
此时在数据库中会产生多个冗杂信息:user_name、id、password;
二、可以将数据库中相同的数据进行归纳到一个表中以避免冗杂:
create table user_info(
id char(36) primary key,
user_name varchar(30) not null,
password varchar(30) not null
)
insert into user_info (id,user_name,password) values ('51b28fe1-4ebf-41ac-a17b-d5e276861fd0','fuliuqingfeng','123456');
create table address(
id char(36) primary key,
user_info_id char(36),
real_name varchar(8) not null,
mobile char(11) not null,
address varchar(150) not null
)
insert into address (id,user_info_id,real_name,mobile,address)
values ('bfb9472a-7911-4e6f-a479-3b719454ebab','51b28fe1-4ebf-41ac-a17b-d5e276861fd0','张三','18920120206','河南安阳');
insert into address (id,user_info_id,real_name,mobile,address)
values ('5227c6b9-45a2-44aa-8ac0-1f63a38d3b65','51b28fe1-4ebf-41ac-a17b-d5e276861fd0','李四','18617297545','北京海淀');
insert into address (id,user_info_id,real_name,mobile,address)
values ('30b8584b-aa6a-4516-a623-03f487058586','51b28fe1-4ebf-41ac-a17b-d5e276861fd0','王五','17694976949','山西大同');
此时可以通过user_info中的id遍历用户信息中重复的数据,可是存在以下问题:
此时address表中中可以添加一些与用户id无关的数据:此时造成脏数据
insert into address (id,user_info_id,real_name,mobile,address)
values ('hhh','啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊','王五','17694976949','山西大同');
像上面的代码是可以被执行的
三、解决方案:外键的确定
create table user_info(
id char(36) primary key,
user_name varchar(30) not null,
password varchar(30) not null
)
insert into user_info (id,user_name,password) values ('51b28fe1-4ebf-41ac-a17b-d5e276861fd0','fuliuqingfeng','123456');
create table address(
id char(36) primary key,
user_info_id char(36),
real_name varchar(8) not null,
mobile char(11) not null,
address varchar(150) not null,
constraint address_user_info_id_fk foreign key(user_info_id) references user_info(id)
)
insert into address (id,user_info_id,real_name,mobile,address)
values ('bfb9472a-7911-4e6f-a479-3b719454ebab','51b28fe1-4ebf-41ac-a17b-d5e276861fd0','张三','18920120206','河南安阳');
insert into address (id,user_info_id,real_name,mobile,address)
values ('5227c6b9-45a2-44aa-8ac0-1f63a38d3b65','51b28fe1-4ebf-41ac-a17b-d5e276861fd0','李四','18617297545','北京海淀');
insert into address (id,user_info_id,real_name,mobile,address)
values ('30b8584b-aa6a-4516-a623-03f487058586','51b28fe1-4ebf-41ac-a17b-d5e276861fd0','王五','17694976949','山西大同');
此时在address表中添加:
constraint address_user_info_id_fk foreign key(user_info_id) references user_info(id)
此时指定address中的(user_info_id)为外键,作用:
1、不可以在再添加外键数据中输出与外键所对应的主键不同的内容
2、不可以删除外键所指向的表(外键所在的表没有数据时可以删除)