有两张表,班级表(classinfo)，学生成绩表(student);
我要查询每个班级成绩最高的和成绩最低的学员的班级编号，学员编号，学员成绩，还有查询每个班级的平均成绩
create table ClassInfo (
classID int identity(1,1) primary key ,
classNmae varchar(20)
)
create table students(
sNo int identity(1,1) primary key,
sName varchar(20),
scord int,
classid int foreign key references ClassInfo(classID)
)
insert into ClassInfo values('一班');
insert into ClassInfo values('二班');
insert into ClassInfo values('三班');
insert into students values('张三',60,1);
insert into students values('张四',70,1)
insert into students values('张五',80,1)
insert into students values('张六',90,1)
insert into students values('张三2',60,2);
insert into students values('张四2',70,2)
insert into students values('张五2',80,2)
insert into students values('张六2',75,2)
insert into students values('张三3',89,3);
insert into students values('张四3',70,3)
insert into students values('张五3',80,3)
insert into students values('张六3',75,3)
select a.classID ,b.scord from ClassInfo as a,students as b left join students as c where b.classID=c.c
