SQL Server 基础语法3(超详细!)

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统计所有学生平均分

use school 
select AVG(grade) as 平均分
from SC

统计每个学生的总分、平均分、课程门数

select sno as 学号,SUM(grade) as 总分,AVG(grade) as 平均分,COUNT(sno) 门数
from SC
group by sno

求学号为“20010101”学生的总分和平均分

select sno,SUM(grade) as 总分,AVG(grade) as 平均分
from SC
where sno=20010101
group by sno

求选修“C1”号课程的最高分、最低分及之间相差的分数

select MAX(grade) as 最高分,MIN(grade) as 最低分,MAX(grade)-MIN(grade) as 差值
from SC
where cno='C1' --int型无需加引号，varchar型要

求选修“C1”号课程的最高分、最低分及之间相差的分数

select cno as 课程号,MAX(grade) as 最高分,MIN(grade) as 最低分,MAX(grade)-MIN(grade) as 差值
from SC
where cno='C1' --int型无需加引号，varchar型要
group by cno --单个列与聚合函数同时查找时，分组需要按照这个列来分组

求选修“C1”号课程的学生人数和最高分

select COUNT(sno) 选课人数,MAX(grade) as 最高分
from SC                                                                         
where cno='C1'

求“计算机系”学生的总数

select COUNT(distinct sno) as 总数
from Student
where sdept='计算机'

统计有成绩同学的人数

select COUNT(distinct sno) as 人数 --如果不加distinct，那么会把不同课程的成绩，但是学号相同的算作多个人
from SC
where grade is not NULL
--where grade!=NULL --是空（is NULL）或不是空（is not NULL),没有等不等于NULL

3.8求女学生总数和平均年龄

select COUNT(distinct sno) as 女生人数,AVG(sage) as 平均年龄
from Student
where sex='女'

按系来统计女学生的人数

select sdept as 系,COUNT(distinct sno) as 女生人数
from Student
where sex='女'--出现在条件中的列不能出现在查询中
group by sdept

查询选修两门以上课程的学生的学号和选课门数

select sno as 学号,COUNT(cno) as 选课门数
from SC
group by sno 
--where COUNT(cno)>=2 where语句的子句不能用聚合类型
having COUNT(cno)>=2

查询平均成绩大于70分的课程号和平均成绩

select cno as 课程号,AVG(grade) as 平均成绩
from SC
group by cno
having AVG(grade)>70

查询选修“C1”的学生学号和成绩，并按成绩降序排列

select sno as 学号,grade as 成绩
from SC
where cno='C1'
order by grade desc

查询选修“C2、C3、C4”或“C5”课程的学号、课程号和成绩，

 --查询结果按学号升序排列，学号相同再按成绩降序排列cx
select sno as 学号,cno as 课程号,grade as 成绩
from SC
where cno in('C2','C3','C4','C5')
order by sno asc,grade desc--asc升序，desc降序

求有三门以上选课成绩及格的学生的学号及其总成绩，查询结果按总成绩降序列出

select  sno as 学号,SUM(grade) as 总分
from SC
where grade>=60
group by sno
having COUNT(cno)>3
order by SUM(grade) desc