zoukankan      html  css  js  c++  java
  • 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.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)
    --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

  • 相关阅读:
    C# 图解教程 第一章 C#和.NET框架
    How I explained OOD to my wife(转)
    ListView 无 DataSource 依然用 DataPager 翻页
    【树莓派】crontab的两个问题
    【CentOS 7】scp示例
    【CentOS 7】nginx配置web服务器
    【CentOS_7】安装nginx
    【python 2.7】获取外部参数
    【python 2.7】输入任意字母数字,输出其对应的莫尔斯码并播放声音
    【python 2.7】python读取json数据存入MySQL
  • 原文地址:https://www.cnblogs.com/Zzzzn/p/10150148.html
Copyright © 2011-2022 走看看