SQL练习1关于插入删除，修改，单表查询

--创建数据库
create database studentsDB

--创建表
create table student
(
 id int primary key,
 stuid char(10),
 stuname char(20),
 classname char(20),
 addresss char(40),
 cols int,
 sex char(10)
)

--更改数据项的数据类型
alter table student
alter column cols float

--插入数据
insert into student(id,stuid,stuname,classname,addresss,cols,sex)
values(1,'001','张三','1','重庆',100,'男')

insert into student(id,stuid,stuname,classname,addresss,cols,sex)
values(2,'002','李四','2','四川',70,'女')

insert into student(id,stuid,stuname,classname,addresss,cols,sex)
values(3,'003','李丽','1','贵州',99,'男')

insert into student(id,stuid,stuname,classname,addresss,cols,sex)
values(4,'004','王五','3','武汉',55,'女')

insert into student(id,stuid,stuname,classname,addresss,cols,sex)
values(5,'005','陈好','1','香港',67,'男')

--查询分数在60-100的信息
select id,stuid,stuname,classname,addresss,cols,sex from student where cols between 60 and 100

--查询班级里姓李的同学的信息
select id,stuid,stuname,classname,addresss,cols,sex from student where stuname like '李%'

--查询分数等于99和100的同学的信息
select id,stuid,stuname,classname,addresss,cols,sex from student where cols = 99 or cols =100

--根据分数降序排列
select id,stuid,stuname,classname,addresss,cols,sex from student order by cols desc

--根据性别求出男生和女生分数的平均值
select avg(cols) as 平均成绩,sex from student group by sex

--求出各班的平均成绩
select avg(cols) as 平均成绩,classname from student group by classname