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)
create table service( id int primary key auto_increment, name varchar(20), host varchar(15) not null, port int not null, unique(host,port) #联合唯一 );mysql> insert into service values-> (1,'nginx','192.168.0.10',80),-> (2,'haproxy','192.168.0.20',80),-> (3,'mysql','192.168.0.30',3306)-> ; Query OK, 3 rows affected (0.01 sec) Records: 3 Duplicates: 0 Warnings: 0mysql> insert into service(name,host,port) values('nginx','192.168.0.10',80); ERROR 1062 (23000): Duplicate entry '192.168.0.10-80' for key 'host'
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: 0mysql> 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)
#在创建完表后,修改自增字段的起始值 mysql> create table student(-> id int primary key auto_increment,-> name varchar(20),-> sex enum('male','female') default 'male'-> );mysql> alter table student auto_increment=3;mysql> show create table student; ....... ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8mysql> insert into student(name) values('egon'); Query OK, 1 row affected (0.01 sec)mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 3 | egon | male | +----+------+------+ row in set (0.00 sec)mysql> show create table student; ....... ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8#也可以创建表时指定auto_increment的初始值,注意初始值的设置为表选项,应该放到括号外 create table student( id int primary key auto_increment, name varchar(20), sex enum('male','female') default 'male' )auto_increment=3;#设置步长 sqlserver:自增步长基于表级别create table t1(id int。。。)engine=innodb,auto_increment=2 步长=2 default charset=utf8mysql自增的步长:show session variables like 'auto_inc%';#基于会话级别set session auth_increment_increment=2 #修改会话级别的步长#基于全局级别的set global auth_increment_increment=2 #修改全局级别的步长(所有会话都生效)#!!!注意了注意了注意了!!! If the value of auto_increment_offset is greater than that of auto_increment_increment, the value of auto_increment_offset is ignored. 翻译:如果auto_increment_offset的值大于auto_increment_increment的值,则auto_increment_offset的值会被忽略 比如:设置auto_increment_offset=3,auto_increment_increment=2mysql> set global auto_increment_increment=5; Query OK, 0 rows affected (0.00 sec)mysql> set global auto_increment_offset=3; Query OK, 0 rows affected (0.00 sec)mysql> show variables like 'auto_incre%'; #需要退出重新登录 +--------------------------+-------+ | Variable_name | Value | +--------------------------+-------+ | auto_increment_increment | 1 | | auto_increment_offset | 1 | +--------------------------+-------+create table student( id int primary key auto_increment, name varchar(20), sex enum('male','female') default 'male' );mysql> insert into student(name) values('egon1'),('egon2'),('egon3'); mysql> select * from student; +----+-------+------+ | id | name | sex | +----+-------+------+ | 3 | egon1 | male | | 8 | egon2 | male | | 13 | egon3 | male | +----+-------+------+
六 foreign key
一 快速理解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,必须保证唯一 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 | +----+-------+--------+
二 如何找出两张表之间的关系
分析步骤: #1、先站在左表的角度去找 是否左表的多条记录可以对应右表的一条记录,如果是,则证明左表的一个字段foreign key 右表一个字段(通常是id)#2、再站在右表的角度去找 是否右表的多条记录可以对应左表的一条记录,如果是,则证明右表的一个字段foreign key 左表一个字段(通常是id)#3、总结: #多对一: 如果只有步骤1成立,则是左表多对一右表 如果只有步骤2成立,则是右表多对一左表#多对多 如果步骤1和2同时成立,则证明这两张表时一个双向的多对一,即多对多,需要定义一个这两张表的关系表来专门存放二者的关系#一对一: 如果1和2都不成立,而是左表的一条记录唯一对应右表的一条记录,反之亦然。这种情况很简单,就是在左表foreign key右表的基础上,将左表的外键字段设置成unique即可
三 建立表之间的关系
#一对多或称为多对一 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) ;
一夫多妻制#妻子表的丈夫id外键到丈夫表的id
#多对多 三张表:出版社,作者信息,书多对多: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) ;
单张表:用户表+相亲关系表,相当于:用户表+相亲关系表+用户表 多张表:用户表+用户与主机关系表+主机表中间那一张存放关系的表,对外关联的字段可以联合唯一
#一对一 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, qq varchar(10) not null, phone char(16) not null );create table student( id int primary key auto_increment, class_name varchar(20) not null, 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,qq,phone) values ('李飞机','31811231',13811341220), ('王大炮','123123123',15213146809), ('守榴弹','283818181',1867141331), ('吴坦克','283818181',1851143312), ('赢火箭','888818181',1861243314), ('战地雷','112312312',18811431230) ;#增加学生 insert into student(class_name,customer_id) values ('脱产3班',3), ('周末19期',4), ('周末19期',5) ;
例一:一个用户只有一个博客用户表:id name1 egon2 alex3 wupeiqi博客表 fk+uniqueid url name_id1 xxxx 12 yyyy 33 zzz 2例二:一个管理员唯一对应一个用户用户表:id user password1 egon xxxx2 alex yyyy管理员表:fk+uniqueid user_id password1 1 xxxxx2 2 yyyyy