常用的网站开发语言,wordpress+读取excel,互联网行业前景分析,太原搜索引擎推广阅读目录 一 介绍二 not null与default三 unique四 primary key五 auto_increment六 foreign key七 作业一 介绍 约束条件与数据类型的宽度一样#xff0c;都是可选参数 作用#xff1a;用于保证数据的完整性和一致性主要分为#xff1a; PRIMARY KEY (PK) 标识该字段为该… 阅读目录 一 介绍二 not null与default三 unique四 primary key五 auto_increment六 foreign key七 作业 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 doesnt have a default valuedefault
#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 |
----------------------------- 验证 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 View Code 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) not nullunique的化学反应 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 联合唯一 四 primary key 从约束角度看primary key字段的值不为空且唯一那我们直接使用not nullunique不就可以了吗要它干什么 主键primary key是innodb存储引擎组织数据的依据innodb称之为索引组织表一张表中必须有且只有一个主键。 一个表中可以 单列做主键多列做主键复合主键 单列做主键
#方法一not nullunique
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_namemysql 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) 单列主键 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 多列主键 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) View Code 了解知识 #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_increment3;mysql show create table student;
.......
ENGINEInnoDB AUTO_INCREMENT3 DEFAULT CHARSETutf8mysql 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;
.......
ENGINEInnoDB AUTO_INCREMENT4 DEFAULT CHARSETutf8#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_increment3;#设置步长
sqlserver自增步长基于表级别create table t1id int。。。engineinnodb,auto_increment2 步长2 default charsetutf8mysql自增的步长show session variables like auto_inc%;#基于会话级别set session auth_increment_increment2 #修改会话级别的步长#基于全局级别的set global auth_increment_increment2 #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 这相当于第一步步子就迈大了扯着了蛋
比如设置auto_increment_offset3auto_increment_increment2mysql set global auto_increment_increment5;
Query OK, 0 rows affected (0.00 sec)mysql set global auto_increment_offset3;
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 |
----------------- 步长:auto_increment_increment,起始偏移量:auto_increment_offset 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
)engineinnodb;#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
)engineinnodb;#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 id3;
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 id22222 where id2;
mysql select * from employee;
-------------------
| id | name | dpt_id |
-------------------
| 1 | egon | 1 |
| 3 | alex2 | 22222 |
| 4 | alex3 | 22222 |
| 5 | alex1 | 22222 |
------------------- 示范 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)
; View Code 一夫多妻制#妻子表的丈夫id外键到丈夫表的id 其他例子 #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)
; View Code 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 其他例子 #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 keyunique #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)
; View Code 例一一个用户只有一个博客用户表id name1 egon2 alex3 wupeiqi博客表 fkuniqueid 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fkuniqueid user_id password1 1 xxxxx2 2 yyyyy 其他例子 七 作业 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usergroupcreate 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businesscreate 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hostcreate 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)
; View Code 作业 转载于:https://www.cnblogs.com/zhanglin123/p/9269406.html
