连接查询和分组查询

一. 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
了，
    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