Mysql表的约束设计和关联关系设计

Mysql表的约束设计和关联关系设计
======================表的完整性======================
(1)实体完整性：每条记录有一个唯一标识符，通常用无任何业务含义的字段表示(主键)

(2)参照完整性：一张表的某个字段必须引用另一张表的某个字段值(外键)

(3)域完整性：域即单元数据，域中的数值必须符合一定的规则

定义主键约束

primary key:不允许为空，不允许重复

删除主键：alter table tablename drop primary key ;

定义主键自动增长(id,没有业务含义)

auto_increment(MySQL特有/UUID类生成)

定义唯一约束

unique

定义非空约束

not null

定义外键约束

constraint ordersid_FKforeign key(ordersid) references orders(id),

2 键的概念

(1)主键：只有唯一字段

(2)组合主键：由多个字段组合起来，形成唯一字段

(3)外键：针对多张表之间的关联

3 主键的特点

(1)主键不能重复

(2)主键不能为NULL

(3)auto_increment是MySQL特有的，默认从1开始，该ID值与表同生亡

(4)多人项目中，通常使用UUID来生成唯一的主键值，便于多个合并数据时依然保持实体完整性



--id主键

mysql> create table teacher(id intprimary key,

-> name varchar(20),

-> brithday date);

Query OK, 0 rows affected (0.61 sec)

mysql> select * from teacher;

Empty set (0.00 sec)

mysql> insert into teachervalues(1,'jack','1992-2-12');

Query OK, 1 row affected (0.11 sec)

mysql> select * from teacher;

+----+------+------------+

| id | name | brithday |

+----+------+------------+

| 1| jack | 1992-02-12 |

+----+------+------------+

1 row in set (0.00 sec)

mysql> desc teacher;

+----------+-------------+------+-----+---------+-------+

| Field | Type | Null | Key |Default | Extra |

+----------+-------------+------+-----+---------+-------+

| id | int(11) | NO | PRI | NULL | |

| name | varchar(20) | YES | | NULL | |

| brithday | date | YES | | NULL | |

+----------+-------------+------+-----+---------+-------+

3 rows in set (0.00 sec)

--主键不能重复,主键冲突,1代表第一列，即id列

mysql> insert into teachervalues(1,'marry','1996-2-12');

ERROR 1062 (23000): Duplicate entry '1' forkey 'PRIMARY'

mysql>

--主键不能为null

mysql> insert into teachervalues(null,'marry','1996-2-12');

ERROR 1048 (23000): Column 'id' cannot benull

-- 删除主键，主键在表只有一个，要么是一列，要么是多列

mysql> alter table teacher drop primarykey;

Query OK, 1 row affected (2.13 sec)

Records: 1 Duplicates: 0 Warnings: 0

mysql> desc teacher;

+----------+-------------+------+-----+---------+-------+

| Field | Type | Null | Key |Default | Extra |

+----------+-------------+------+-----+---------+-------+

| id | int(11) | NO | | NULL | |

| name | varchar(20) | YES | | NULL | |

| brithday | date | YES | | NULL | |

+----------+-------------+------+-----+---------+-------+

3 rows in set (0.01 sec)

--主键id自动增长

mysql> drop table if exists teacher;

Query OK, 0 rows affected (0.32 sec)

mysql> create table teacher(id intprimary key auto_increment,

-> name varchar(20),

-> birthday date);

Query OK, 0 rows affected (0.96 sec)

mysql> desc teacher;

+----------+-------------+------+-----+---------+----------------+

| Field | Type | Null | Key |Default | Extra |

+----------+-------------+------+-----+---------+----------------+

| id | int(11) | NO | PRI | NULL |
auto_increment |

| name | varchar(20) | YES | | NULL | |

| birthday | date | YES | | NULL | |

+----------+-------------+------+-----+---------+----------------+

3 rows in set (0.01 sec)

mysql> insert into teacher(name,birthday)values('jack','1992-02-12');

Query OK, 1 row affected (0.06 sec)

mysql> insert intoteacher(name,birthday) values('marry','1992-4-8');

Query OK, 1 row affected (0.09 sec)

mysql> insert intoteacher(name,birthday) values('arry','1982-4-8');

Query OK, 1 row affected (0.14 sec)

mysql> select * from teacher;

+----+-------+------------+

| id | name | birthday |

+----+-------+------------+

| 1| jack | 1992-02-12|

| 2| marry | 1992-04-08 |

| 3| arry | 1982-04-08|

+----+-------+------------+

3 rows in set (0.00 sec)

--检查主键id增长的序列

mysql> delete from teacher whereid<=3;

Query OK, 3 rows affected (0.08 sec)

mysql> select * from teacher;

Empty set (0.00 sec)

mysql> insert intoteacher(name,birthday) values('jack','1992-02-12');

Query OK, 1 row affected (0.13 sec)

mysql> select * from teacher;

+----+------+------------+

| id | name | birthday |

+----+------+------------+

| 4| jack | 1992-02-12 |

+----+------+------------+

1 row in set (0.00 sec)

--多人项目中，通常使用UUID来生成唯一的主键值，便于多个合并数据时依然保持实体完整性

4 唯一约束的特点

(1)非NULL值不能重复

(2)可以插入多个NULL值

(3)'NULL'空串和NULL是不同的概念

mysql> drop table teacher;

Query OK, 0 rows affected (0.42 sec)

--唯一

mysql> create table teacher(id intprimary key auto_increment,

-> name varchar(20) unique,

-> birthday date);

Query OK, 0 rows affected (1.49 sec)

mysql> insert intoteacher(name,birthday) values('marry','2011-2-2');

Query OK, 1 row affected (0.11 sec)

mysql> insert intoteacher(name,birthday) values(NULL,'2011-1-1');

Query OK, 1 row affected (0.04 sec)

mysql> desc teacher;

+----------+-------------+------+-----+---------+----------------+

| Field | Type | Null | Key |Default | Extra |

+----------+-------------+------+-----+---------+----------------+

| id | int(11) | NO | PRI | NULL | auto_increment |

| name | varchar(20) | YES |
UNI | NULL | |

| birthday | date | YES | | NULL | |

+----------+-------------+------+-----+---------+----------------+

3 rows in set (0.00 sec)

--违反唯一

mysql> insert intoteacher(name,birthday) values('marry','2011-2-2');

ERROR 1062 (23000): Duplicate entry 'marry'for key 'name'

5 非空约束特点

(1)不能插入NULL值

(2)主键约束=非NULL约束+唯一约束

--违返不为空约束

mysql> drop table if exists teacher;

Query OK, 0 rows affected (0.19 sec)

mysql> create table teacher(

-> id int primary keyauto_increment,

-> name varchar(20) not nullunique,

-> birthday date

-> );

Query OK, 0 rows affected (1.45 sec)

mysql> insert intoteacher(name,birthday) values(NULL,'2011-1-1');

ERROR 1048 (23000): Column 'name' cannot benull

6 外健特点

(1)外键值必须来源于所引用别一个表主键值，或NULL



*7 关联关系

(1)一对一(外健根业务有关)

(2)一对多或多对一(外键放置在多方)

(3)多对多(外健放置在关联表中，即将一个多对多拆分成二个一对多关系)

一对一关系一:



mysql> create table person(

-> id int primary keyauto_increment,

-> name varchar(20) not null

-> );

Query OK, 0 rows affected (1.16 sec)

mysql> insert into person(name)values('jack');

Query OK, 1 row affected (0.15 sec)

mysql> insert into person(name)values('marry');

Query OK, 1 row affected (0.06 sec)

--card中pid为外person的外键

mysql> create table card(

-> id int primary key auto_increment,

-> location varchar(20) notnull,

-> pid int,

-> constraint pid_FK foreignkey(pid) references person(id)

-> );

Query OK, 0 rows affected (1.55 sec)

mysql> insert into card(location,pid)values('BJ',1);

Query OK, 1 row affected (0.11 sec)

mysql> insert into card(location,pid)values('GZ',2);

Query OK, 1 row affected (0.39 sec)

--外键pid可以插入NULL

mysql> insert into card(location,pid)values('CS',NULL);

Query OK, 1 row affected (0.05 sec)

--显示person与card表中的数据

mysql> select * from person;

+----+-------+

| id | name |

+----+-------+

| 1| jack |

| 2| marry |

+----+-------+

2 rows in set (0.00 sec)

mysql> select * from card;

+----+----------+------+

| id | location | pid |

+----+----------+------+

| 1| BJ | 1 |

| 2| GZ | 2 |

| 3| CS | NULL |

+----+----------+------+

3 rows in set (0.00 sec)

--插入pid为3的出错误，外键pid必须来源另一表的主键值或为NULL

mysql> insert into card(location,pid)values('NJ',3);

ERROR 1452 (23000): Cannot add or update achild row: a foreign key constraint fails (`mydb2`.`card`, CONSTRAINT `pid_FK`FOREIGN KEY (`pid`) REFERENCES `person` (`id`))

注意:外键值必须来源于所引用别一个表主键值，或NULL

--有外键关系的行不能删除，需先删除其关联的外键关系,再进行删除主键关系的数据，与创建的时候相反

mysql> deletefrom person where name='jack';

ERROR 1451(23000): Cannot delete or update a parent row: a foreign key constraint fails(`mydb2`.`card`, CONSTRAINT `pid_FK` FOREIGN KEY (`pid`) REFERENCES `person`(`id`))

一对一关系方式二:主键当作另一个表的外键

例下面的card表的id既是主键又是外键



mysql> drop table if exists card;

Query OK, 0 rows affected (0.22 sec)

mysql> drop table if exists person;

Query OK, 0 rows affected (0.22 sec)

mysql> create table person(

-> id int primary key auto_increment,

-> name varchar(20) not null

-> );

Query OK, 0 rows affected (0.90 sec)

mysql> insert into person(name)values('jack');

Query OK, 1 row affected (0.07 sec)

mysql> insert into person(name)values('marry')

-> ;

Query OK, 1 row affected (0.10 sec)

--使用主键作为外键

mysql> create table card(

-> id int primary keyauto_increment,

-> location varchar(20) notnull,

-> constraint id_FK foreignkey(id) references person(id)

-> );

Query OK, 0 rows affected (0.86 sec)

mysql> insert into card(location)values('BJ');

Query OK, 1 row affected (0.06 sec)

mysql> insert into card(location)values('CS');

Query OK, 1 row affected (0.18 sec);

--id为3,
不符合参照完整性,3在person表中打不到

mysql> insert into card(location) values('GZ');

ERROR 1452 (23000): Cannot add or update achild row: a foreign key constraint fails (`mydb2`.`card`, CONSTRAINT `id_FK`FOREIGN KEY (`id`) REFERENCES `person` (`id`))

--NULL也不行，因为这里id为一个非空的属性

mysql> insert into card(location)values(NULL);

ERROR 1048 (23000): Column 'location'cannot be null

==========================一对多关系=============================

设计方案:

一方做一个容器，多方做一个引用



mysql> create table department(

-> id int primary keyauto_increment,

-> name varchar(20) not null

-> );

Query OK, 0 rows affected (1.70 sec)

mysql> insert into department(name)values('软件部');

Query OK, 1 row affected (0.10 sec)

mysql> insert into department(name)values('销售部');

Query OK, 1 row affected (0.06 sec)

mysql> create table employee(

-> id int primary keyauto_increment,

-> name varchar(20) not null,

-> did int,

-> constraint did_FK foreignkey(did) references department(id)

-> );

Query OK, 0 rows affected (1.02 sec)

mysql> insert into employee(name,did)values('jack',1);

Query OK, 1 row affected (0.12 sec)

mysql> insert into employee(name,did)values('marry',1);

Query OK, 1 row affected (0.13 sec)

mysql>

mysql> select * from department;

+----+-----------+

| id | name |

+----+-----------+

| 1| 软件部 |

| 2| 销售部 |

+----+-----------+

2 rows in set (0.00 sec)

mysql> select * from employee;

+----+-------+------+

| id | name | did |

+----+-------+------+

| 1| jack | 1 |

| 2| marry | 1 |

+----+-------+------+

2 rows in set (0.00 sec)

--查询软件部的员工(利用子查询)

mysql> select * from employee wheredid=(select id from department where name='软件部');

+----+-------+------+

| id | name | did |

+----+-------+------+

| 1| jack | 1 |

| 2| marry | 1 |

+----+-------+------+

2 rows in set (0.05 sec)

--利用多表查询

mysql> select e.name as 员工姓名,d.name as 部门名称 from employee e,department d where d.name='软件部';

+--------------+--------------+

| 员工姓名 | 部门名称 |

+--------------+--------------+

| jack | 软件部 |

| marry | 软件部 |

+--------------+--------------+

2 rows in set (0.01 sec)

============================多对多方案=============================

--创建的时候，先创建两边的，再创建中间的

--删除的时候先删除中间的，再删除两边的



mysql> drop table if exists middle;

Query OK, 0 rows affected, 1 warning (0.00sec)

mysql> drop table if exists student;

Query OK, 0 rows affected (0.25 sec)

mysql> drop table if exists teacher;

Query OK, 0 rows affected (0.33 sec)

mysql> create table if not existsstudent(

-> id int primary keyauto_increment,

-> name varchar(20) not null

-> );

Query OK, 0 rows affected (1.19 sec)

mysql> insert into student(name)values('jack');

Query OK, 1 row affected (0.08 sec)

mysql> insert into student(name)values('marry');

Query OK, 1 row affected (0.04 sec)

mysql> create table if not existsteacher(

-> id int primary key auto_increment,

-> name varchar(20) not null

-> );

Query OK, 0 rows affected (1.43 sec)

mysql> insert into teacher(name)values('赵');

Query OK, 1 row affected (0.34 sec)

mysql> insert into teacher(name)values('蔡');

Query OK, 1 row affected (0.07 sec)

--多对多(外健放置在关联表中，即将一个多对多拆分成二个一对多关系)

-- 组合主键

mysql> create table if not existsmiddle(

-> sid int,

-> tid int,

-> constraint sid_FK foreignkey(sid) references student(id),

-> constraint tid_FK foreignkey(tid) references teacher(id),

-> primary key(sid,tid)

-> );

Query OK, 0 rows affected (1.59 sec)

mysql> insert into middle(sid,tid)values(1,1);

Query OK, 1 row affected (0.14 sec)

mysql> insert into middle(sid,tid)values(1,2);

Query OK, 1 row affected (0.06 sec)

mysql> insert into middle(sid,tid)values(2,1);

Query OK, 1 row affected (1.07 sec)

mysql> insert into middle(sid,tid)values(2,2);

Query OK, 1 row affected (0.07sec)mysql> insert into middle(sid,tid) values(2,2);

--查询赵教师教过的所有学员,组合查询

--t.name='赵'
条件

-- and m.sid=s.id andm.tid=t.id
--表连接起来

模式：

select 列出需要显示的字段

from 列出所涉及到的所有表，建议写别名

where 业务条件and
表关联条件

--sql

mysql> selectt.name as 老师, s.name as 学员

-> from teacher as t,student as s,middleas m

-> where t.name = '赵'and m.sid=s.id and m.tid=t.id;

+--------+--------+

| 老师 | 学员 |

+--------+--------+

| 赵 | jack |

| 赵 | marry |

+--------+--------+

2 rows in set(0.00 sec)