创建数据库

create database db1;

删除数据库

drop database db1;

创建数据表

create table tb1 用户表 (
    id int not null auto_increment primary key,
    name char(10),
    department_id int,
    p_id int)engine=innodb default charset=utf8;

主键（primary key）一个表只能有一个主键，主键可以由一列或者多列组成

外键的创建

CREATE TABLE t5 (nid int(11) NOT NULL AUTO_INCREMENT,
    pid int(11) not NULL,
    num int(11),
    primary key(nid,pid) 

数据行的操作

数据的插入

insert into tb1(name,age) values('ax',8);
insert into tb12(name,age) select name,age from tb11;

表中数据的删除

delete from t1;
truncate table t1;
drop table t1;
delete from tb1 where id > 10;
delete from tb12 where id >=2 or name='alex';

数据的更新

update tb1 set name='root' where id > 10

数据的查询

select * from tb;
select id,name from tb;`

表结构的查看

show create table t1;
desc t1;

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;
select * from tb12 limit 0,10;
select * from tb12 limit 10,10;
select * from tb12 limit 20,10;
select * from tb12 limit 10 offset 20;  
# page = input('请输入要查看的页码')  
# page = int(page)  
# (page-1) * 10  
# select * from tb12 limit 0,10; 1  
# select * from tb12 limit 10,10;2

排序

select * from tb12 order by id desc; 大到小
select * from tb12 order by id asc;  小到大
select * from tb12 order by age desc,id desc;  
  
取后 10 条数据  
select * from tb12 order by id desc limit 10;

分组

select count(id),max(id),part_id from userinfo5 group by part_id;
聚合函数有下面几个：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;

自增值设置

表自增值的设置

alter table t1 auto_increment=20;-- 这个就表示从开始 20 开始算，用上面的 show create table t1\G 就可以看到当前的值是多少。

基于会话级别

-- 查看当前的会话值
show session variables like 'auto_incre%'

-- 设置会话步长
set session auto_increment_increment=2;

-- 设置起始值
set session auto_increment_offset=10;

基于全局设置

-- 查看全局的设置值
show global variables like 'auto_inc%';

-- 设置全局步长值
set global auto_increment_increment=3;

-- 设置起始值
set global auto_increment_offset=11;

sql server 是在创建表的时候就可以自己设置，灵活度很高 REATE 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

唯一索引

create table t1(
    id int,
    num int,
    xx int,
    unique qu1 (num ,xx) -- 意思就是这两列在一行上面数据不能相同，例如都是 1,1, 就不行
);

唯一索引：约束不能重复（可以为空）主键索引：约束不能重复（不可以为空）他们的特点都是加速查询

外键一对一

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;

外键多对多

 示例 1：用户表
    相亲表

示例 2：用户表
    主机表
    用户主机关系表
===》多对多

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;

连表操作

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.ti;

select count(id) from userinfo5;