分组查询
1.语法
SELECT ... FROM ... GROUP BY ...
--分组查询Group by
select COUNT(*) as 学生人数,GradeId as 年级编号 from Student group by GradeId
2.进行多表之间的分组查询
--查询每门课程的平均分
--where 和 group by 和 order by 的顺序
select AVG(StudentResult) as 平均分,SubjectName as 课程名称 from Subject,Result
where Subject.SubjectId=Result.SubjectId group by Subject.SubjectName order by 平均分
3.多列分组
--查询每学期男女同学的总人数
select COUNT(*) as 学生人数,GradeId as 年级编号,Sex as 性别 from Student group by GradeId,Sex
4.使用having字句帮助group by进行分组条件筛选，注意：条件必须在聚合函数内或者查询的分组条件内
--查询学生总人数超过5人的年级
select COUNT(*) as 学生人数,GradeId as 年级编号 from Student group by GradeId having COUNT(*) >5
二. 内连接 inner join on
1.内连接可以不用区分表与表之间的顺序，但是要指定好关系
2.内连接可以将inner省略
3.通where语句效果相同
select stu.StudentName,res.StudentResult,res.SubjectId from Student as stu inner join Result as res on (stu.StudentNo=res.StudentNo)
4.在查询不同记录时，会将表与表中的记录数相乘然后减去相同的记录数
select stu.StudentName,res.StudentResult,res.SubjectId from Result as res inner join Student as stu on (stu.StudentNo<>res.StudentNo)
例如：12*10-10
三. 外连接
1.左外连接 LEFT JOIN 左边的表作为主表
select StudentName,StudentResult,Student.StudentNo from Student LEFT JOIN Result on Student.StudentNo=Result.StudentNo
2.右外连接 RIGHT JOIN 右边的表作为主表
select StudentName,StudentResult,Student.StudentNo from Student RIGHT JOIN Result on Student.StudentNo=Result.StudentNo
