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  • 连接查询和分组查询

    一、分组查询

    --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)

    二、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

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  • 原文地址:https://www.cnblogs.com/wws553/p/10169849.html
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