SQL课堂笔记--嵌套查询

Posted 2020-10-14

     2017.11.15
六:嵌套查询
 嵌套查询概述:
  一个select-from-where 语句称为一个查询块
  将一个查询块嵌套在另一个查询块的where子句或having短语的条件中的查询称为嵌套查询
 
 
 例1:查询选修了2号课程得学生姓名:
  内连接:select sname from student,sc where student.sno=sc.sno and cno=2
        或:select sname from student where sno in   --外层查询/父查询
     (select sno from sc where cno=2)   --内层查询/子查询
  
 引出子查询得谓词:
   带有in谓词的子查询  
   带有比较运算符的子查询
   带有any或all谓词的子查询
   带有exists谓词的子查询
 1.带有in谓词的子查询 

  例2:查询与‘刘晨‘在同一个专业学习的学生:(涉及2张表)
   select sno,sname,sdept from student where sdept in (select sdept from student where sname=‘刘晨‘)

      查询与‘刘晨‘在同一个专业学习的学生但是不包括‘刘晨‘:
   select sno,sname,sdept from student where sdept in (select sdept from student where sname=‘刘晨‘) and sname<>‘刘晨‘
 
  例3:查询选修了课程名为‘信息系统‘的学生学号和姓名:(涉及3张表)
   select sno,sname from student where sno in  --3.最后在student关系中取出sno和sname
    ( select sno from sc where cno in  --2.然后在sc中找出选修了课程的学号和姓名
     (select cno from course where cname=‘信息系统‘));  --1.首先在course关系中找出‘信息系统‘的课程号
   select sno,sname from student,sc,course where student.sno=sc.sno....

 2.带有比较运算符的子查询

  例4:查询与‘刘晨‘在同一个专业学习的学生: 
   select sno,sname,sdept from student where sdept = (select sdept from student where sname=‘刘晨‘)

 3.带有any或all谓词的子查询:
  
  >any  >min
  >all
  <any          <max
  <all
  >=any  >=min
  >=all
  <=any         <=max
  <=all
  =any
  =all
  !=(<>)any      不存在
  !=(<>)all not in
  
  例5:查询其他专业中比计算机任意一个(其中某一个)学生年龄小的学生姓名和年龄
   select sname,sage from student where sage<any(select sage from student where sdept=‘计算机‘) and sdept<>‘计算机‘;

  例6:查询其他专业中比计算机任意一个(其中某一个)学生年龄小的学生姓名和年龄(用聚合函数):
   select sname,sage from student where sage<(select (max)sage from student where sdept=‘计算机‘) and sdept<>‘计算机‘;

  例7:查询...

 4.带有exists谓词的子查询:
  1.exists谓词:
   不返回任何数据,只产生逻辑真值‘true‘或逻辑假值‘false‘
    若内层查询结果非空,则返回真值
    若内层查询结果为空,则返回假值 
   由exists引出的子查询,其目标列表达式通常都用*,因为带exists的子查询只返回真或假值,给出列名五实际意义

  2.not exists谓词:
   不返回结果集为真
   

  例8:查询所有选修了1号课程的学生姓名:
   select sname from studnet where sno in (select sno from sc where cno=1)
   select sname from studnet where exists(select * from sc where sc.sno=student.sno and cno=‘1‘ )

  例9:查询没有选修1号课程的学生姓名:
   select sname from studnet where not exists(select * from sc where sc.sno=student.sno and cno=‘1‘)

   查询与‘刘晨‘在同一个专业学习的学生:
   select sno,sname,sdept from studnet s1 where exists (select * from student s2 where s2.sdept=s1.sdept and sname=‘刘晨‘)

  查询选修了全部课程的学生姓名:
    法1:select sname from student where not exists
     (select * from course where not exists
       (select * from sc where sc.sno=studnet.sno and sc.sno=course.cno))
   
    法2:select sname from student where sno in
     (select sno from sc group by sno having count(*)=(select count(*) from course))

 

练习:

use students
go
--1、用嵌套查询所有比“王敏”大的学生的姓名和年龄。
select sname,sage from student where sage >all (select sage from student where sname=‘王敏‘)
 
--2、用嵌套查询选修了3号课程的学生姓名。
select sname from student where sno in (select sno from sc where cno=3)

--3、用嵌套查询选修了课程名为“数学”的学生学号和姓名。
select sno,sname from student where sno in (select sno from sc where sdept=‘数学‘)

--4、查询其他系中比电子系任意一个学生年龄小的学生姓名和年龄。
select  sname,sage from student where sage<any (select sage from student where sdept=‘电子‘) and sdept<>‘电子‘

--5、查询其他系中比电子系所有学生年龄都小的学生姓名及年龄。
-select sname,sage from student where sage <all(select sage from student where sdept=‘电子‘) and sdept<>‘电子‘;
select sname,sage from student where sage <(select MIN(sage) from student where sdept=‘电子‘) and sdept<>‘电子‘;

--6、用EXISTS嵌套查询所有选修了1号课程的学生姓名。
select sname from student where exists(select * from sc where sc.sno=student.sno and cno=1)
 
--7、查询至少选修了一门课程的学生姓名。
--select sname from student where not exists(select * from sc where sc.sno=student.sno and sdept=null)
select sname from student where sno in (select sno from sc)

--8、查询选修了两门及以上课程的学生姓名。
select sname from student where sno in (select sno from sc group by sc.sno having count(*)>=2 )
--Select sname from student a join sc b on a.sno=b.sno where having count(b.sno)>=2

--9、查询没有选修任何课程的学生姓名。
select sname from student where not exists(select * from sc where student.sno=sc.sno)
--或select sname from student where not in (select sno from sc )

--10、查询至少被选修了二次的课程号及课程名称。
select cno,cname from course where cno in (select cno from sc group by cno having count(*)>=2 )

--11、查询选修了全部课程的学生姓名。
select sname from student where not exists
     (select * from course where not exists
       (select * from sc where sc.sno=student.sno and sc.sno=course.cno))

--12、查询所有学生都选修了的课程信息。
select cno,cname from course where not exists
     (select * from student where not exists
      (select * from sc where sc.sno=student.sno and sc.cno=course.cno))