关系
多对多的关系，如何通过mysql来表示
站在老师的角度
一个老师可以教多个学生，
一个老师也可以教一个学生。
站在学生的角度
一个学生可以被一个老师教
一个学生也可以被多个老师教
结论：如果站在两边看都是一对多的情况，那么这个关系就是多对多的。
问题：
如果表示a老师教过x学生和y学生，x学生和y学生同时也被b老师教
解决方法
多对多关系，无论是把外键放在哪一张表都不合适，因为可能有多个值
解决方案：建立一个中间的关系表
create table student(
id int primary key auto_increment,
name char(10)
) charset utf8;
create table teacher(
id int primary key auto_increment,
name char(10)
) charset utf8;
create table t_s_r(
id int primary key auto_increment,
t_id int,
s_id int,
foreign key(t_id) references teacher(id),
foreign key(s_id) references student(id)
) charset utf8 ;
insert into teacher values(null,"bgon"),(null,"nike");
insert into student values(null,"老王"),(null,"老李");
# 老王被bgon教过
insert into t_s_r values(null,1,1);
# nike教过老李
insert into t_s_r values(null,2,2);
# nike教过老王
insert into t_s_r values(null,2,1);
# 现在已知老师名称为bgon,请找出他教过的那些学生
mysql> select id from teacher where name="bgon";
+----+
| id |
+----+
| 1 |
+----+
1 row in set (0.00 sec)
mysql> select s_id from t_s_r where t_id=1;
+------+
| s_id |
+------+
| 1 |
+------+
1 row in set (0.00 sec)
mysql> select name from student where id=1;
+--------+
| name |
+--------+
| 老王 |
+--------+
# 子查询
mysql> select name from student where id=(select s_id from t_s_r where t_id=(select id from teacher where name="bgon"));
+--------+
| name |
+--------+
| 老王 |
+--------+
1 row in set (0.00 sec)
# 已知学生名为老李，请查询出哪些老师教过他.
mysql> select id from student where name = "老李";
+----+
| id |
+----+
| 2 |
+----+
1 row in set (0.00 sec)
mysql> select t_id from t_s_r where s_id=2;
+------+
| t_id |
+------+
| 2 |
+------+
1 row in set (0.00 sec)
mysql> select name from teacher where id=2;
+------+
| name |
+------+
| nike |
+------+
1 row in set (0.00 sec)
# 子查询
mysql> select name from teacher where id=(select t_id from t_s_r where s_id=(select id from student where name = "老李"));
+------+
| name |
+------+
| nike |
+------+
1 row in set (0.00 sec)
总结：
如何确认多对多的关系？
站在两个表的角度去想
处理方式，通过在两个表中间建立一个外键表，该外键表分别都关联两表的字段。
联合唯一约束
对于上面的t_s_r表进行改进
# 原表
create table t_s_r(
id int primary key auto_increment,
t_id int,
s_id int,
foreign key(t_id) references teacher(id),
foreign key(s_id) references student(id),
) charset utf8 ;
# 改进方法一(表已创建的情况下)：unique key
alter table t_s_r add unique key(t_id,s_id);
# 改进方法二(表没创建的情况下):unique key
create table t_s_r(
id int primary key auto_increment,
t_id int,
s_id int,
foreign key(t_id) references teacher(id),
foreign key(s_id) references student(id),
unique key(t_id,s_id)
) charset utf8 ;
# 联合主键
create table t_s_r(
id int primary key auto_increment,
t_id int,
s_id int,
foreign key(t_id) references teacher(id),
foreign key(s_id) references student(id),
primary key(t_id,s_id)
) charset utf8 ;
对于上面的t_s_r表进行改进------最终版
# 创建学生表
create table student(
id int primary key auto_increment,
name char(10)
) charset utf8;
# 创建教师表
create table teacher(
id int primary key auto_increment,
name char(10)
) charset utf8;
# 创建学生和老师关系表
create table t_s_r(
t_id int,
s_id int,
foreign key(t_id) references teacher(id),
foreign key(s_id) references student(id),
primary key(t_id,s_id)
) charset utf8 ;
insert into teacher values(null,"bgon"),(null,"nike");
insert into student values(null,"老王"),(null,"老李");
# 老王被bgon教过
insert into t_s_r values(null,1,1);
# nike教过老李
insert into t_s_r values(null,2,2);
# nike教过老王
insert into t_s_r values(null,2,1);
# 问题：现在已知老师名称为bgon,请找出他教过的那些学生
mysql> select id from teacher where name="bgon";
+----+
| id |
+----+
| 1 |
+----+
1 row in set (0.00 sec)
mysql> select s_id from t_s_r where t_id=1;
+------+
| s_id |
+------+
| 1 |
+------+
1 row in set (0.00 sec)
mysql> select name from student where id=1;
+--------+
| name |
+--------+
| 老王 |
+--------+
# 整合查询语句：子查询
mysql> select name from student where id=(select s_id from t_s_r where t_id=(select id from teacher where name="bgon"));
+--------+
| name |
+--------+
| 老王 |
+--------+
1 row in set (0.00 sec)
# 问题：已知学生名为老李，请查询出哪些老师教过他.
mysql> select id from student where name = "老李";
+----+
| id |
+----+
| 2 |
+----+
1 row in set (0.00 sec)
mysql> select t_id from t_s_r where s_id=2;
+------+
| t_id |
+------+
| 2 |
+------+
1 row in set (0.00 sec)
mysql> select name from teacher where id=2;
+------+
| name |
+------+
| nike |
+------+
1 row in set (0.00 sec)
# 整合查询语句：子查询
mysql> select name from teacher where id=(select t_id from t_s_r where s_id=(select id from student where name = "老李"));
+------+
| name |
+------+
| nike |
+------+
1 row in set (0.00 sec)
一对一关系
例如：每一个人都有一个身份证。一个身份证只对应一个人
分表：
垂直分表，例如：人物的详细信息，就可以垂直分表
# 全表
create table person(
id int primary key auto_increment,
name char(10),
age int,
height float,
weigth float
)
# 垂直分表：person
create table person(
id int primary key auto_increment,
name char(10),
age int
) charset utf8;
# 垂直分表：person_info
create table person_info(
id int primary key,
height float,
weigth float,
foreign key(id) references person(id)
) charset utf8;
水平分表
数据量很大，使用一个表，查询效率低，使用两个表来存取这些数据
处理一对一关系处理方式：
先确定先后顺序
将先存在的数据作为主表
后存在的作为从表
使两个表的id保持一一对应
方法1：从表的id即是主键又是外键
方法2：从表的id设置为外键，并保证唯一
人物关系表，从客户演变为学生：
# 创建客户表
create table kehu_t(
id int primary key auto_increment,
name char(10),
phone char(11)
)
# 创建学生表
create table student_t(
id int primary key auto_increment,
card_id char(18)
)
为什么要分表：
数据分担在多个表，提高了查询的效率
