《第十一章：连接查询和分组》

--切换数据库

use MySchool

--查询对应年级下的学生人数

SELECT COUNT(1) AS '人数' FROM Student WHERE GradeId=1
SELECT COUNT(1) AS '人数' FROM Student WHERE GradeId=2
SELECT COUNT(1) AS '人数' FROM Student WHERE GradeId=3

--分组查询地区对应的人数
--分组查询原理：根据特定列进行分组，找列的相同值

SELECT COUNT(1) AS '人数',Address AS '地区' FROM Student WHERE Address LIKE '北京%' GROUP BY Address

--查询科目的平均分

--WHERE -->GROUP BY -->ORDER BY
SELECT AVG(StudentResult) AS '平均分',Subject.SubjectName FROM Subject,Result WHERE Subject.SubjectId=Result.SubjectId
GROUP BY Subject.SubjectName
ORDER BY AVG(StudentResult) ASC

--查询每学期的男女同学人数
--根据多列进行分组

SELECT COUNT(1) AS '人数',GradeId,Sex FROM Student GROUP BY GradeId,Sex ORDER BY COUNT(1)

--查询科目的平均分大于70

--HAVING和WHERE有什么不同

--where是针对表中原有的记录进行条件筛选

--group by是针对where条件筛选后的记录进行分组

--having是针对group by分组后的记录进行条件筛选

SELECT AVG(StudentResult) AS '平均分',Subject.SubjectName FROM Subject,Result WHERE Subject.SubjectId=Result.SubjectId
GROUP BY Subject.SubjectName HAVING AVG(StudentResult)>=70
ORDER BY AVG(StudentResult)

--连接查询
--1.内连接

SELECT S.StudentName,G.GradeName FROM Student AS S INNER JOIN Grade AS G ON S.GradeId=G.GradeId

--查询学生姓名对应科目和考试分数
--当使用内连接进行多表联查时，直接空格继续JINNER JOIN

SELECT S.StudentName,SU.SubjectName,R.StudentResult FROM Result AS R INNER JOIN Subject AS SU ON (R.SubjectId=SU.SubjectId)
INNER JOIN Student AS S ON (S.StudentNo=R.StudentNo)

--查询学生成绩大于等于65分的记录
--使用连接查询为了指定表与表之间的关系，如果想添加条件那么在指定完关系后加WHERE

SELECT S.StudentName,SU.SubjectName,R.StudentResult FROM Result AS R INNER JOIN Subject AS SU ON (R.SubjectId=SU.SubjectId)
INNER JOIN Student AS S ON (S.StudentNo=R.StudentNo) WHERE R.StudentResult>=65

--2.外连接
--2.1左外链接

SELECT S.StudentName,R.StudentResult,R.SubjectId FROM Result AS R LEFT JOIN Student AS S ON (S.StudentNo=R.StudentNo)
SELECT S.StudentName,G.GradeName FROM Grade AS G LEFT JOIN Student AS S ON(S.GradeId=G.GradeId)
SELECT R.*,S.SubjectName FROM Subject AS S LEFT JOIN Result AS R ON(R.SubjectId=S.SubjectId)

--2.2右外连接

SELECT S.StudentName,R.StudentResult,R.SubjectId FROM Result AS R RIGHT JOIN Student AS S ON (S.StudentNo=R.StudentNo)
SELECT S.StudentName,G.GradeName FROM Student AS S RIGHT JOIN Grade AS G ON(S.GradeId=G.GradeId)
SELECT R.*,S.SubjectName FROM Result AS R RIGHT JOIN Subject AS S ON(R.SubjectId=S.SubjectId)