sql 基础语法3：分组，聚合函数，having，联合查询，快速备份，内联函数

select * from Classinfo
select * from StuInfo
select * from CourseInfo
select * from ScoreInfo

--分组 group by,分组后在结果列中只能出现分组依据列和聚合列
--统计男女人数
select stuSexy,COUNT(*)
from StuInfo
group by stuSexy--分组也是一个聚合过程，把所有性别相同的元组放到了同一行

--算出每门课的平均分
select cId,avg(score)
from ScoreInfo
group by cId --根据科目进行分组，算出平均分

select cId,avg(score)
from ScoreInfo
group by cId,StuId --按多个属性进行分组

--求每个班的男女人数,同时输出班级名
select ci.className,si.stuSexy,COUNT(*)
from StuInfo as si
inner join Classinfo as ci on si.classId=ci.classId
group by ci.className,si.stuSexy --除了这两个分组关键属性，其他属性都被合并了，所以不能再使用

select ci.classId,si.stuSexy,COUNT(*)
from StuInfo as si
inner join Classinfo as ci on si.classId=ci.classId
group by ci.classId,si.stuSexy --只能使用ci.classId和si.stuSexy了

--统计学生编号>2的各班级的各性别的学生人数
select ci.className,si.stuSexy,COUNT(*)
from StuInfo as si
inner join Classinfo as ci on si.classId=ci.classId
where si.stuId>2
group by si.stuSexy,ci.className

--统计学生编号>2的编号为1的班级的各性别的学生人数
select ci.className,si.stuSexy,COUNT(*)
from StuInfo as si
inner join Classinfo as ci on si.classId=ci.classId
where si.stuId>2 and ci.classId=1
group by si.stuSexy,ci.className

--having查询
--统计学生编号>2的编号各班级的各性别的学生人数>1的信息
select ci.className,si.stuSexy,COUNT(*)
from StuInfo as si
inner join Classinfo as ci on si.classId=ci.classId
where si.stuId>2
group by si.stuSexy,ci.className having count(*)>1 --对count增加条件

select * from Classinfo
select * from StuInfo
select * from CourseInfo
select * from ScoreInfo

--联合查询：将多个查询的结果集合合并成一个集合union
--要求：1，结果集列数要一致 2，对应列的类型要一致
--union,union all,except,intersect 并,直接相加，差，交
select stuId from StuInfo
union
select stuid from ScoreInfo

select stuId from StuInfo
union all
select stuid from ScoreInfo

select stuId from StuInfo
except
select stuid from ScoreInfo

select stuId from StuInfo
intersect
select stuid from ScoreInfo

--快速备份
--向一个不存在的表中快速插入数据：包括插入属性行
select * into test1 from Classinfo

--新建一个空表，但是包含属性列
select * into test2 from Classinfo
where 1=2

--向一个存在的表中插入数据
insert into test2(className)--因为classId是自增的所以不能从外界添加，所以只能添加className
select className from Classinfo

--格式转化函数CAST,CONVERT
select CAST(89.000000 as decimal(4,1))
select CONVERT(decimal(4,1),89.000000)

select CAST(1 as CHAR(1))+''

select char(65)
select ASCII('A')
select left('ABCDEF',5)
select right('ABCDEF',5)
select substring('ABCDEF',1,5)
select lower('ABCDEF')
select upper('ABCDEFa')
select ltrim('  ab c')--去掉左侧空格
select RTRIM('abc   ')
select STR(1)

select GETDATE()
select dateAdd(YY,1,GETDATE())
select dateAdd(MM,1,GETDATE())
select dateAdd(DD,1,GETDATE())
select DATEPART("Dayofyear",GETDATE())--求出一年中第几天
select DATEPART("year",GETDATE())--返回int型
select DATEPART("month",GETDATE())
select DATEPART("day",GETDATE())

select dateDiff(DD,dateAdd(YY,1,GETDATE()),GETDATE())--日期差

select STR(DATEPART(YY,stuBirthday))+'-'+ltrim(STR(DATEPART(MM,stuBirthday)))+'-'+ltrim(STR(DATEPART(DD,stuBirthday)))
from StuInfo

select * from Classinfo
select * from StuInfo
select * from CourseInfo
select * from ScoreInfo
--查询科目名称，平均分
select ci.cName,AVG(sco.score)
from ScoreInfo as sco
inner join CourseInfo as ci on sco.scoreId=ci.cId
group by ci.cName

--查询班级名称，一个班平均分
-- sco-stu-class
select ci.className,AVG(sco.score)
from ScoreInfo as sco
inner join StuInfo as si on sco.StuId=si.stuId
inner join Classinfo as ci on si.classId=ci.classId
group by ci.className

--查询查询班级名称，科目名称，平均分
-- course-sco-stu-class
select ci.className,cour.cName,AVG(sco.score)
from ScoreInfo as sco
inner join StuInfo as si on si.stuId=sco.StuId
inner join Classinfo as ci on si.classId=ci.classId
inner join CourseInfo as cour on sco.cId=cour.cId
group by ci.className,cour.cName

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