连接查询和分组查询
1.在SQL Server语言中叫做分组查询，分组查询采用Group BY 子句实现。
二，多列分组查询
2.分组查询还可以按照多个列来进行分组。
使用Group BY 关键字时，是有限制的，在SELECT 列表中可以指定列是有限制的，仅允许以下几项。
被分组的列为每个分组返回一个值，
--查询年级所拥有的人数 select GradeId as 年级,COUNT(Phone) as 人数 from Student
group by GradeId
--根据性别进行分组 select Sex as 性别,COUNT(*) as 人数
from Student group by Sex
--查询每门课程的平均分 select SubjectId as 课程编号,AVG(StudentResult) as 平均分
from Result group by SubjectId
--按地区分类，查询地区的人数 select COUNT(*) as 人数,Address as 地址
from Student group by Address
--查询每门课程的平均分，并且按照分数由低到高的顺序排列显示 select SubjectId as 课程编号,AVG(StudentResult) as 平均分
from Result group by SubjectId order by AVG(StudentResult) desc
--统计每学期男女同学的人数 select COUNT(*) as 人数,GradeId as 年级,Sex as 性别
from Student Group by GradeId,Sex order by GradeId --性别：男和女 年级：1，2，3
--如何获得总人数超过2人的年级 select COUNT(*) as 人数,GradeId as 年级,Sex as 性别
from Student Group by GradeId,Sex having COUNT(*)>=2 order by GradeId
--出生日期大于1990年的学生,获得总人数超过2人的年级 select COUNT(*) as 人数,GradeId as 年级,Sex as 性别
from Student where BornDate >'1990/01/01' Group by GradeId,Sex having COUNT(*)>=2 order by GradeId
--同时从这两个表中取得数据 select Student.StudentName as 姓名,Result.StudentResult as 成绩, Result.SubjectId AS 科目编号
from Student,Result where Student.StudentNo=Result.StudentNo
表连接
select 列 from 表1，表2
where 条件（表1.主键列=表2.外键列）
内连接（inner join）
select 列 from 表1 inner join 表2
on 条件（表1.主键列=表2.外键列）
as也可以为表赋别名
select S.StudentName as 姓名,R.StudentResult as 成绩, R.SubjectId AS 科目编号
from Result as R inner join Student as S on(S.StudentNo=R.StudentNo)
slect S.StudentName as 姓名,R.StudentResult as 成绩, SU.SubjectName AS 科目名称
from Result as R inner join Student as S on(S.StudentNo=R.StudentNo) inner join Subject as SU on(R.SubjectId=SU.SubjectId)
select Student.StudentName as 姓名,Result.StudentResult as 成绩, Subject.SubjectName AS 科目名称
from Student,Result,Subject where Student.StudentNo=Result.StudentNo and Result.SubjectId=Subject.SubjectId
--左外连接
左外连接（left join）
select 列
from 主表
left outer join 从表
on （表1.主键列=表2.外键列）
select S.StudentName,R.SubjectId,R.StudentResult
From Student AS S LEFT JOIN Result as R on(S.StudentNo=R.StudentNo)
--右外连接
右外连接（right join）
select 列
from 从表
right outer join 主表
on （表1.主键列=表2.外键列）
select S.StudentName,R.SubjectId,R.StudentResult
From Result AS R RIGHT JOIN Student as S on(S.StudentNo=R.StudentNo)