连接查询和分组查询

--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.科目名称 FROM Subject,Result WHERE Subject.所属年级=Result.SubjectId GROUP BY Subject.科目名称 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) --内连接和外连接的区别：内连接表与表之间的次序没有分别，指定表与表之间的关系，外链接表顺序有区别，会投影出NULL记录