--自关联
--1 北京 null
--2 朝阳区 1
--3 房山区 1
--4 吉林省 null
--5 长春市 4
--6 吉林市 4
--空间换时间
--商品表
-- id name pic
--商品样式表
-- id pid style pic
--给班级表哦增加主键
alter table my_class add primary
key(name);
--插入数据
insert into my_class values(
'python1907','B408';)
insert into my_class values(
'python1907','B407';)
insert into my_class values(
'python1907','B407';)
--冲突处理
on duplicate key update
--更新教室
room='B407';
insert into my_class values(
'python1903','B408';)
--主键冲突:替换
replace into my_class values('python1903','B406');
--复制创建表
create table my_copy like
my_class;
--删除主键
alter table my_copy drop primary
key;
--虫复制
insert into my_copy select *from
my_class;
insert into my_copy select *from
my_copy;
--更新部分B406变成A406
update my_copy set eoom ='A406'
where room ='B406' limit3;
--删除数据:限制记录数为5
deletefrom my_copy where room=
'B409' limit5;
--给学生表增加主题
aiter table my_student modify id
int primary key auto_increment;
--清空表,重置自增长
truncate my_atudent;
--以下三句有什么区别？
-- deletefrom 表名;
-- truncate表名;
-- drop table表名;
-- select选项
select *from my_copy;
selete all *from my_copy;
--去重
selete distinct *from my_copy;
--向学生表插入数据
insert into my_student
values(null,'bc20190001','张三'，'男'),
(null,'bc20190002','李四','男'),
(null,'bc20190003','王五','女'),
(null,'bc20190004','赵六','男'),
(null,'bc20190005','周七','男');
--字段别名
select id,
numberas 学号,
nameas 姓名,
sex性别 from my_student;
--多表数据源
select *from my_student,my_class;
--子查询
select *from (select *from
my_student)as s;
--增加age年龄和height身高字段
alter table my_student add age
tinyint unsgned;
alter table my_student add height
tinyint unsigned;
--
增加字段值:rand取得一个0-1之间的随机数,floor向下取整
update my_student set age=flooe(
rand()*20+20), height=floor(rand
()20+170),
--找学生id为1,3,5的学生
select *from my_student where id
=1 || id=3 || id=5; --逻辑判断
select *from my_student where id
in(1,3,5); -- in表示在集合中
--找出身高在180到190之间的学生
select *from my_student where
height>=180 and height<=190;
select *from my_student where
height between180 and 190;
select *from my_student where
height between190 and 180;
--不成立,相当于height>=190 and
height<=108
select *from my_student where1;
--所有条件都满足
--根据性别分组
select *from my_student group by
sex;
--
分组统计:身高高矮,平均年龄,总年龄
select sex,count(*),max(height),
min(height),avg(age),sum(age)
from my_student group by sex;
--修改id为4的记录,把年龄置办为NULL
update my_student set age=null
where id=4;
select sex,count(*),count(age),
max(height),min(height),avg(age),
sum(age)from my_student group by
sex;
--修改id为1的记录,把性别置为女
update my_student set sex='女'
where id=1;
-- nan
-- nv
select sex,count(*)count(age),
max(height),min(height),avg(age),
sum(age)from my_student group by
sex desc;
--删除班级表原主键
alter table my_class drop primary
key;
--给班级表增加主键
alter table my_class add id int
primary key auto_increment;
--给学生表增加班级ID
alter table my_student add c_id
int;
update my_student set c_id=ceil(rand()*3);
--多字段分组:先班组,后男女
select c_id,sex,count(*)from
my_student group by c_id,sex;
--多字符排序
--统计
select c_id,count(*)from
my_student group by c_id;
--回溯统计
select c_id,count(*)from
my_student group by c_idwith
rollup;
--多字符分组回溯统计
select c_id,sex,count(*),
group_concat(name)from
my_student group by c_id,sex;
--多字段排序
select c_id,sex,count(*),
group_concat(name)from
my_student group by c_id,sex;
--多字段排序
elect c_id,sex,count(*),
group_concat(name)from
my_student group by c_id,sexwith
rollup;
--
求出所有班级人数大于等于2的学生人数
select c_id,count(*)from
my_student group by c_id having
count(*)>=2;
select c_id,count(*)from
my_student where count(*)>=2
group by c_id; --错误
select c_id,count(*)as total
frommy_student group by c_id having
total>=2
select c_id,count(*)astotal
from my_student where count(*)>=2
group by c_id; --错误
-- having子句进行条件查询
select nameas 名字,numberas
学号 from my_student having名字
like'张%';
--排序
select *from my_student group by
c_id; --分组,为了进行统计
select *from my_student order by
c_id; --排序
--
多字段排序:先班级排序,后性别排序
select *from my-student order by
c_id,sex desc;
--查询学生:前两个
select *from my_student limit2;
select *from my_student limit0,
2; --记录数是从0开始编号
select *from my_student limit2,
2;
select *from my_student limit4,
2;
--更改ID为班级表的第一列
alter table my_class change id id
int first;
--交叉连接
select *from my-student cross
join my_class; -- mu_student
cross join my_class是数据源
--内连接
select *from my_student inner
join my_class on my_student.c_id=
my_class.id;
select *from my_student inner
join my_class on c_id=my_class.id;
select *from my_student inner
join my_class on c_id=id; --
错误,因为两张表都有id字段
--字段和表别名
select s.*,nameas c_name, c.roomfrom my_studentas s
inner join my_classas c
on s.c_id=c_id;
--
把学生表id为5的记录的c_id设置为NULL
update my_student set c_id=null
where id=5;
-- where代替on
select s.*,c.nameas c_name,c.
room --字段别名
from my_studentas s --表别名
inner join my_classas c
where s.c_id=c_id;
--左连接
select s.*,c.nameas c_name,c.
room --字段别名
from my_studentas s
left join my_classas c --
左表为主表:最终记录数至少不少于左表己有的记录数
on s.c_id=c.id;
--右连接
select s.*,c.nameas c_name,c.
room --字段别名
from my_studentas s
height join my_classas c --
右表为主表:最终记录数至少不少于右表己有的记录数
on s.c_id=c_id;
select s.*,c.nameas c_name,c.room
--字段别名
from my_classas c
right join my_studentas s
on s.c_id=c_id;
--自然内连接
select *from my_student natural
join my_class;
--修改班级表的name字段名为c_name
alter table my_class change name
c_name varchar(20)not null;
--自然左外连接
select *from my_student natural
left join my_class;
--外连接模拟自然外连接:using
select *from my_student left
join my_class using(id;
