以下是學習筆記：

1，分組統計

2，分組後篩選統計

select 班級=StudentClass.ClassName,人數=COUNT(*),C#最高分=Max(CSharp),DB最高分=MAX(SQLServerDB),
AVG(CSharp) as C#平均分,AVG(SQLServerDB) as DB平均分
from Students
inner Join StudentClass on Students.ClassId =StudentClass.ClassId
inner join ScoreList on ScoreList.StudentId=Students.StudentId
group by ClassName --ClassName 按照班級的名稱分組


select 班級=StudentClass.ClassName,人數=COUNT(*),C#最高分=Max(CSharp),DB最高分=MAX(SQLServerDB),
AVG(CSharp) as C#平均分,AVG(SQLServerDB) as DB平均分
from Students
inner Join StudentClass on Students.ClassId =StudentClass.ClassId
inner join ScoreList on ScoreList.StudentId=Students.StudentId
group by ClassName --ClassName 按照班級的名稱分組
having AVG(CSharp)>=70 and AVG(SQLServerDB)>=70 --分組統計後不能有where條件篩選，要用having

3，重複資料問題

--在知道那個欄位重複的情況
select StudentId from ScoreList  group by StudentId having COUNT(*)>1

--查詢所有重複的記錄
select * from ScoreList
where StudentId in(select StudentId from ScoreList  group by StudentId having COUNT(*)>1)
order by StudentId

--其他方法
select * from ScoreList
where (select COUNT(*) from ScoreList  s where s.StudentId=ScoreList.StudentId)>1
order by StudentId

--過濾掉重複資料
select distinct StudentId,CSharp from ScoreList

select distinct StudentId,CSharp,SQLServerDB from ScoreList