本文谈谈MySQL的开发必会的sql语句

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) --这里就是把两列设置成了主键
                    ) ENGINE=InnoDB DEFAULT CHARSET=utf8;

                    create table t6(
                        id int auto_increment primary key,
                        name char(10),
                        id1 int,
                        id2 int,
                        CONSTRAINT fk_t5_t6 foreign key (id1,id2) REFERENCES t1(nid,pid) --这里是设置外键
                    )engine=innodb default charset=utf8;

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;

表中的数据的删除

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 t1G 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;