约束
一 介绍
约束条件与数据类型的宽度一样，都是可选参数
作用：用于保证数据的完整性和一致性
主要分为：
PRIMARY KEY (PK) 标识该字段为该表的主键，可以唯一的标识记录
FOREIGN KEY (FK) 标识该字段为该表的外键
NOT NULL 标识该字段不能为空
UNIQUE KEY (UK) 标识该字段的值是唯一的
AUTO_INCREMENT 标识该字段的值自动增长（整数类型，而且为主键）
DEFAULT 为该字段设置默认值
UNSIGNED 无符号
ZEROFILL 使用0填充
说明：
1. 是否允许为空，默认NULL，可设置NOT NULL，字段不允许为空，必须赋值 2. 字段是否有默认值，缺省的默认值是NULL，如果插入记录时不给字段赋值，此字段使用默认值 sex enum('male','female') not null default 'male' age int unsigned NOT NULL default 20 必须为正值（无符号） 不允许为空 默认是20 3. 是否是key 主键 primary key 外键 foreign key 索引 (index,unique...)
二 not null与default
是否可空，null表示空，非字符串
not null - 不可空
null - 可空
默认值，创建列时可以指定默认值，当插入数据时如果未主动设置，则自动添加默认值
create table tb1(
nid int not null defalut 2,
num int not null
)
==================not null==================== mysql> create table t1(id int); #id字段默认可以插入空 mysql> desc t1; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | +-------+---------+------+-----+---------+-------+ mysql> insert into t1 values(); #可以插入空 mysql> create table t2(id int not null); #设置字段id不为空 mysql> desc t2; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | NO | | NULL | | +-------+---------+------+-----+---------+-------+ mysql> insert into t2 values(); #不能插入空 ERROR 1364 (HY000): Field 'id' doesn't have a default value ==================default==================== #设置id字段有默认值后，则无论id字段是null还是not null，都可以插入空，插入空默认填入default指定的默认值 mysql> create table t3(id int default 1); mysql> alter table t3 modify id int not null default 1; ==================综合练习==================== mysql> create table student( -> name varchar(20) not null, -> age int(3) unsigned not null default 18, -> sex enum('male','female') default 'male', -> hobby set('play','study','read','music') default 'play,music' -> ); mysql> desc student; +-------+------------------------------------+------+-----+------------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+------------------------------------+------+-----+------------+-------+ | name | varchar(20) | NO | | NULL | | | age | int(3) unsigned | NO | | 18 | | | sex | enum('male','female') | YES | | male | | | hobby | set('play','study','read','music') | YES | | play,music | | +-------+------------------------------------+------+-----+------------+-------+ mysql> insert into student(name) values('egon'); mysql> select * from student; +------+-----+------+------------+ | name | age | sex | hobby | +------+-----+------+------------+ | egon | 18 | male | play,music | +------+-----+------+------------+
三 unique
============设置唯一约束 UNIQUE=============== 方法一： create table department1( id int, name varchar(20) unique, comment varchar(100) ); 方法二： create table department2( id int, name varchar(20), comment varchar(100), constraint uk_name unique(name) ); mysql> insert into department1 values(1,'IT','技术'); Query OK, 1 row affected (0.00 sec) mysql> insert into department1 values(1,'IT','技术'); ERROR 1062 (23000): Duplicate entry 'IT' for key 'name'
mysql> create table t1(id int not null unique); Query OK, 0 rows affected (0.02 sec) mysql> desc t1; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | +-------+---------+------+-----+---------+-------+ 1 row in set (0.00 sec)
四 primary key
primary key字段的值不为空且唯一
一个表中可以：
单列做主键
多列做主键（复合主键）
但一个表内只能有一个主键primary key
============单列做主键=============== #方法一：not null+unique create table department1( id int not null unique, #主键 name varchar(20) not null unique, comment varchar(100) ); mysql> desc department1; +---------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | NO | UNI | NULL | | | comment | varchar(100) | YES | | NULL | | +---------+--------------+------+-----+---------+-------+ rows in set (0.01 sec) #方法二：在某一个字段后用primary key create table department2( id int primary key, #主键 name varchar(20), comment varchar(100) ); mysql> desc department2; +---------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | YES | | NULL | | | comment | varchar(100) | YES | | NULL | | +---------+--------------+------+-----+---------+-------+ rows in set (0.00 sec) #方法三：在所有字段后单独定义primary key create table department3( id int, name varchar(20), comment varchar(100), constraint pk_name primary key(id); #创建主键并为其命名pk_name mysql> desc department3; +---------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | YES | | NULL | | | comment | varchar(100) | YES | | NULL | | +---------+--------------+------+-----+---------+-------+ rows in set (0.01 sec)
==================多列做主键================ create table service( ip varchar(15), port char(5), service_name varchar(10) not null, primary key(ip,port) ); mysql> desc service; +--------------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------------+-------------+------+-----+---------+-------+ | ip | varchar(15) | NO | PRI | NULL | | | port | char(5) | NO | PRI | NULL | | | service_name | varchar(10) | NO | | NULL | | +--------------+-------------+------+-----+---------+-------+ 3 rows in set (0.00 sec) mysql> insert into service values -> ('172.16.45.10','3306','mysqld'), -> ('172.16.45.11','3306','mariadb') -> ; Query OK, 2 rows affected (0.00 sec) Records: 2 Duplicates: 0 Warnings: 0 mysql> insert into service values ('172.16.45.10','3306','nginx'); ERROR 1062 (23000): Duplicate entry '172.16.45.10-3306' for key 'PRIMARY'
五 auto_increment
约束字段为自动增长，被约束的字段必须同时被key约束
#不指定id，则自动增长 create table student( id int primary key auto_increment, name varchar(20), sex enum('male','female') default 'male' ); mysql> desc student; +-------+-----------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+-----------------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(20) | YES | | NULL | | | sex | enum('male','female') | YES | | male | | +-------+-----------------------+------+-----+---------+----------------+ mysql> insert into student(name) values -> ('egon'), -> ('alex') -> ; mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 1 | egon | male | | 2 | alex | male | +----+------+------+ #也可以指定id mysql> insert into student values(4,'asb','female'); Query OK, 1 row affected (0.00 sec) mysql> insert into student values(7,'wsb','female'); Query OK, 1 row affected (0.00 sec) mysql> select * from student; +----+------+--------+ | id | name | sex | +----+------+--------+ | 1 | egon | male | | 2 | alex | male | | 4 | asb | female | | 7 | wsb | female | +----+------+--------+ #对于自增的字段，在用delete删除后，再插入值，该字段仍按照删除前的位置继续增长 mysql> delete from student; Query OK, 4 rows affected (0.00 sec) mysql> select * from student; Empty set (0.00 sec) mysql> insert into student(name) values('ysb'); mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 8 | ysb | male | +----+------+------+ #应该用truncate清空表，比起delete一条一条地删除记录，truncate是直接清空表，在删除大表时用它 mysql> truncate student; Query OK, 0 rows affected (0.01 sec) mysql> insert into student(name) values('egon'); Query OK, 1 row affected (0.01 sec) mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 1 | egon | male | +----+------+------+ 1 row in set (0.00 sec)
偏移量offset
六 foreign key
员工信息表有三个字段：工号 姓名 部门
公司有3个部门，但是有1个亿的员工，那意味着部门这个字段需要重复存储，部门名字越长，越浪费
解决方法：
我们完全可以定义一个部门表
然后让员工信息表关联该表，如何关联，即foreign key
#表类型必须是innodb存储引擎，且被关联的字段，即references指定的另外一个表的字段，必须是主键 create table department( id int primary key, name varchar(20) not null )engine=innodb; #dpt_id外键，关联父表（department主键id），同步更新，同步删除 create table employee( id int primary key, name varchar(20) not null, dpt_id int, constraint fk_name foreign key(dpt_id) references department(id) on delete cascade on update cascade )engine=innodb; #先往父表department中插入记录 insert into department values (1,'欧德博爱技术有限事业部'), (2,'艾利克斯人力资源部'), (3,'销售部'); #再往子表employee中插入记录 insert into employee values (1,'egon',1), (2,'alex1',2), (3,'alex2',2), (4,'alex3',2), (5,'李坦克',3), (6,'刘飞机',3), (7,'张火箭',3), (8,'林子弹',3), (9,'加特林',3) ; #删父表department，子表employee中对应的记录跟着删 mysql> delete from department where id=3; mysql> select * from employee; +----+-------+--------+ | id | name | dpt_id | +----+-------+--------+ | 1 | egon | 1 | | 2 | alex1 | 2 | | 3 | alex2 | 2 | | 4 | alex3 | 2 | +----+-------+--------+ #更新父表department，子表employee中对应的记录跟着改 mysql> update department set id=22222 where id=2; mysql> select * from employee; +----+-------+--------+ | id | name | dpt_id | +----+-------+--------+ | 1 | egon | 1 | | 3 | alex2 | 22222 | | 4 | alex3 | 22222 | | 5 | alex1 | 22222 | +----+-------+--------+
表1 foreign key 表2
则表1的多条记录对应表2的一条记录，即多对一
利用foreign key的原理我们可以制作两张表的多对多，一对一关系
多对多：
表1的多条记录可以对应表2的一条记录
表2的多条记录也可以对应表1的一条记录
一对一：
表1的一条记录唯一对应表2的一条记录，反之亦然
分析时，我们先从按照上面的基本原理去套，然后再翻译成真实的意义，就很好理解了
三张表：出版社，作者信息，书
一对多（或多对一）：一个出版社可以出版多本书
关联方式：foreign key
=====================多对一===================== create table press( id int primary key auto_increment, name varchar(20) ); create table book( id int primary key auto_increment, name varchar(20), press_id int not null, foreign key(press_id) references press(id) on delete cascade on update cascade ); insert into press(name) values ('北京工业地雷出版社'), ('人民音乐不好听出版社'), ('知识产权没有用出版社') ; insert into book(name,press_id) values ('九阳神功',1), ('九阴真经',2), ('九阴白骨爪',2), ('独孤九剑',3), ('降龙十巴掌',2), ('葵花宝典',3) ;
多对多：一个作者可以写多本书，一本书也可以有多个作者，双向的一对多，即多对多
关联方式：foreign key+一张新的表
=====================多对多===================== create table author( id int primary key auto_increment, name varchar(20) ); #这张表就存放作者表与书表的关系，即查询二者的关系查这表就可以了 create table author2book( id int not null unique auto_increment, author_id int not null, book_id int not null, constraint fk_author foreign key(author_id) references author(id) on delete cascade on update cascade, constraint fk_book foreign key(book_id) references book(id) on delete cascade on update cascade, primary key(author_id,book_id) ); #插入四个作者，id依次排开 insert into author(name) values('egon'),('alex'),('yuanhao'),('wpq'); #每个作者与自己的代表作如下 1 egon: 1 九阳神功 2 九阴真经 3 九阴白骨爪 4 独孤九剑 5 降龙十巴掌 6 葵花宝典 2 alex: 1 九阳神功 6 葵花宝典 3 yuanhao: 4 独孤九剑 5 降龙十巴掌 6 葵花宝典 4 wpq: 1 九阳神功 insert into author2book(author_id,book_id) values (1,1), (1,2), (1,3), (1,4), (1,5), (1,6), (2,1), (2,6), (3,4), (3,5), (3,6), (4,1) ;
#两张表：学生表和客户表
一对一：一个学生是一个客户，一个客户有可能变成一个学校，即一对一的关系
关联方式：foreign key+unique
#一定是student来foreign key表customer，这样就保证了： #1 学生一定是一个客户， #2 客户不一定是学生，但有可能成为一个学生 create table customer( id int primary key auto_increment, name varchar(20) not null ); create table student( id int primary key auto_increment, name varchar(20) not null, class_name varchar(20) not null default 'python自动化', level int default 1, customer_id int unique, #该字段一定要是唯一的 foreign key(customer_id) references customer(id) #外键的字段一定要保证unique on delete cascade on update cascade ); #增加客户 insert into customer(name) values ('李飞机'), ('王大炮'), ('守榴弹'), ('吴坦克'), ('赢火箭'), ('战地雷') ; #增加学生 insert into student(name,customer_id) values ('李飞机',1), ('王大炮',2) ;
练习：账号信息表，用户组，主机表，主机组
#用户表 create table user( id int not null unique auto_increment, username varchar(20) not null, password varchar(50) not null, primary key(username,password) ); insert into user(username,password) values ('root','123'), ('egon','456'), ('alex','alex3714') ; #用户组表 create table usergroup( id int primary key auto_increment, groupname varchar(20) not null unique ); insert into usergroup(groupname) values ('IT'), ('Sale'), ('Finance'), ('boss') ; #主机表 create table host( id int primary key auto_increment, ip char(15) not null unique default '127.0.0.1' ); insert into host(ip) values ('172.16.45.2'), ('172.16.31.10'), ('172.16.45.3'), ('172.16.31.11'), ('172.10.45.3'), ('172.10.45.4'), ('172.10.45.5'), ('192.168.1.20'), ('192.168.1.21'), ('192.168.1.22'), ('192.168.2.23'), ('192.168.2.223'), ('192.168.2.24'), ('192.168.3.22'), ('192.168.3.23'), ('192.168.3.24') ; #业务线表 create table business( id int primary key auto_increment, business varchar(20) not null unique ); insert into business(business) values ('轻松贷'), ('随便花'), ('大富翁'), ('穷一生') ; #建关系：user与usergroup create table user2usergroup( id int not null unique auto_increment, user_id int not null, group_id int not null, primary key(user_id,group_id), foreign key(user_id) references user(id), foreign key(group_id) references usergroup(id) ); insert into user2usergroup(user_id,group_id) values (1,1), (1,2), (1,3), (1,4), (2,3), (2,4), (3,4) ; #建关系：host与business create table host2business( id int not null unique auto_increment, host_id int not null, business_id int not null, primary key(host_id,business_id), foreign key(host_id) references host(id), foreign key(business_id) references business(id) ); insert into host2business(host_id,business_id) values (1,1), (1,2), (1,3), (2,2), (2,3), (3,4) ; #建关系：user与host create table user2host( id int not null unique auto_increment, user_id int not null, host_id int not null, primary key(user_id,host_id), foreign key(user_id) references user(id), foreign key(host_id) references host(id) ); insert into user2host(user_id,host_id) values (1,1), (1,2), (1,3), (1,4), (1,5), (1,6), (1,7), (1,8), (1,9), (1,10), (1,11), (1,12), (1,13), (1,14), (1,15), (1,16), (2,2), (2,3), (2,4), (2,5), (3,10), (3,11), (3,12) ;
