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Mysql学习日记-02外键 ,索引, sql语句的补充

时间:2020-07-03 09:11:18      阅读:67      评论:0      收藏:0      [点我收藏+]

标签:date   foreign   ref   ons   delete   名称   结合   arch   显示   

主键:

  一个表只能有一个主键

  主键可以由多列组成

外键:

  就是把一个表的成员 和 另外一个表的成员联系起来

  比如 student 里有(学号,姓名)  而 score里有(学号,成绩) 这个时候就可以把   student(学号) = score (学号)关联起来

  语法规则是 : 

create  table score (

    sid int  not null auto_increment primary  key,

    number iint 

)

create  table student (

     nid   int  not null auto_increment primary  key,

    name   varchar(20),

    constraint fk_xxx  foreign key (nid )  references   score (nid)

  )

 

对于自增补充:

    是对表里面的序号的步长设置
  desc t10;

  show create table t10;

  show create table t10 \G;

  alter table t10 AUTO_INCREMENT=20;

MySQL: 自增步长
  基于会话级别:
  show session variables like ‘auto_inc%‘; 查看全局变量
  set session auto_increment_increment=2; 设置会话步长
  # set session auto_increment_offset=10;
基于全局级别:
  show global variables like ‘auto_inc%‘; 查看全局变量
  set global auto_increment_increment=2; 设置会话步长
  # set global auto_increment_offset=10;

SqlServer:自增步长:
    基础表级别:
      CREATE TABLE `t5` (
        “nid` int(11) NOT NULL AUTO_INCREMENT,
        `pid` int(11) NOT NULL,
        `num` int(11) DEFAULT NULL,
        PRIMARY KEY (`nid`,`pid`)
      ) ENGINE=InnoDB AUTO_INCREMENT=4, 步长=2 DEFAULT CHARSET=utf8

      CREATE TABLE `t6` (
        `nid` int(11) NOT NULL AUTO_INCREMENT,
        `pid` int(11) NOT NULL,
        `num` int(11) DEFAULT NULL,
         PRIMARY KEY (`nid`,`pid`)
        ) ENGINE=InnoDB AUTO_INCREMENT=4, 步长=20 DEFAULT CHARSET=utf8

索引:

               

        create table t1(
           id int ....,
            num int,
            xx int,
          unique 唯一索引名称 (列名,列名),
            constraint ....
            )

       索引即是绑定一对一     加速查找      

        约束constraint不能重复(可以为null)  主键不能重复(不能为null)

外键与索引的结合  :

比如  一个用户只能拥有一个博客     ,一对一

create table userinfo1(                                    
id int auto_increment primary key,
name char(10),
gender char(10),
email varchar(64)
)engine=innodb default charset=utf8;

create table admin(
id int not null auto_increment primary key,
username varchar(64) not null,
password VARCHAR(64) not null,
user_id int not null,
unique uq_u1 (user_id),
CONSTRAINT fk_admin_u1 FOREIGN key (user_id) REFERENCES userinfo1(id)
)engine=innodb default charset=utf8;

多对多    :

     用户表

     主机表

      用户主机关系表

create table userinfo2(
id int auto_increment primary key,
name char(10),
gender char(10),
email varchar(64)
)engine=innodb default charset=utf8;

create table host(
id int auto_increment primary key,
hostname char(64)
)engine=innodb default charset=utf8;


create table user2host(
id int auto_increment primary key,
userid int not null,
hostid int not null,
unique uq_user_host (userid,hostid),
CONSTRAINT fk_u2h_user FOREIGN key (userid) REFERENCES userinfo2(id),
CONSTRAINT fk_u2h_host FOREIGN key (hostid) REFERENCES host(id)
)engine=innodb default charset=utf8;

总结:对sql语句数据行的操作补充 必须注重细节!括号 逗号不要忘

逻辑关系要理顺

create table tb12(
    id int auto_increment primary key,
    name varchar(32),
    age int
)

增  :  

        inset into tb12(name ,age ) values(‘kk‘,12);

       inset into  tb12 (name,age) values(‘kk‘,12),(‘alex‘,13);

      insert into tb12(name,age) select name,age from tb11;

删:

     

delete from tb12;
delete from tb12 where id !=2
delete from tb12 where id =2
delete from tb12 where id > 2
delete from tb12 where id >=2
delete from tb12 where id >=2 or name=‘alex‘

  

改:

 

update tb12 set name=‘alex‘ where id>12 and name=‘xx‘
update tb12 set name=‘alex‘,age=19 where id>12 and name=‘xx‘

 

查:

select * from tb12;

select id,name from tb12;

select id,name from tb12 where id > 10 or name =‘xxx‘;

select id,name as cname from tb12 where id > 10 or name =‘xxx‘;

select name,age,11 from tb12;

其他:

select * from tb12 where id != 1
select * from tb12 where id in (1,5,12);
select * from tb12 where id not in (1,5,12);
select * from tb12 where id in (select id from tb11)
select * from tb12 where id between 5 and 12;

通配符:

select * from tb12 where name like "a%"
select * from tb12 where name like "a_"

 

分页:

select * from tb12 limit 10;(10页)

select * from tb12 limit 0,10;(从第一个开始数,10个)
select * from tb12 limit 10,10;
select * from tb12 limit 20,10;

select * from tb12 limit 10 offset 20;

 

排序:
select * from tb12 order by id desc; 大到小   记忆  D大
select * from tb12 order by id asc; 小到大  记忆 a小
select * from tb12 order by age desc,id desc;

分组:用于数目的统计 比如表中一个学生可以选择多个课程   用group by students 统计下来其实只有一个人

select count(id),max(id),part_id from userinfo5 group by part_id;

count
max
min
sum
avg

**** 如果对于聚合函数结果进行二次筛选时?必须使用having ****
select count(id),part_id from userinfo5 group by part_id having count(id) > 1;

select count(id),part_id from userinfo5 where id > 0 group by part_id having count(id) > 1;

连表操作:

其实可要记住一个就行了  

select * from userinfo5,department5

select * from userinfo5,department5 where userinfo5.part_id = department5.id

select * from userinfo5 left join department5 on userinfo5.part_id = department5.id
select * from department5 left join userinfo5 on userinfo5.part_id = department5.id
# userinfo5左边全部显示


# select * from userinfo5 right join department5 on userinfo5.part_id = department5.id
# department5右边全部显示



select * from userinfo5 innder join department5 on userinfo5.part_id = department5.id
将出现null时一行隐藏


select * from
department5
left join userinfo5 on userinfo5.part_id = department5.id
left join userinfo6 on userinfo5.part_id = department5.id


select
score.sid,
student.sid
from
score

left join student on score.student_id = student.sid

left join course on score.course_id = course.cid

left join class on student.class_id = class.cid

left join teacher on course.teacher_id=teacher.tid

 

Mysql学习日记-02外键 ,索引, sql语句的补充

标签:date   foreign   ref   ons   delete   名称   结合   arch   显示   

原文地址:https://www.cnblogs.com/kangkang1999/p/13228084.html

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