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sql 经典查询50题 思路(一)

将来的事 2019-11-03 14:13:00 阅读数:25 评论数:0 点赞数:0 收藏数:0

因为需要提高一下sql的查询能力,当然最快的方式就是做一些实际的题目了。选择了这个sql的50题,这次大概做了前10题左右,把思路放上来,也是一个总结。

具体题目见:

https://zhuanlan.zhihu.com/p/72223558

 

第一部分的题目主要使用的技术是连表查询和子查询,难倒不难,主要是要把思路转换过来。

首先画出一个各个表之间的关系图(没画图之前关系老是搞不清)

 

 

 1.查询" 01 "课程比" 02 "课程成绩高的学生的信息及课程分数

学生的信息在表1当中,课程成绩在表4当中,当然要用到连表查询。

这里很有普遍性的一个问题是:要从表4中找出Sid相同,但是Cid为1的score大于Cid为2的score的记录。这里要使用子查询,分别限定条件为Cid=‘1’,Cid='2',变成两个表,再查满足条件的就很简单了。

select Student.SId,Student.Sname,Student.Sage,Student.Ssex,r.科目一成绩,r.科目二成绩
from study.dbo.Student 
right join
(select t1.SId as 学生ID,t1.score as 科目一成绩,t2.score as 科目二成绩
from 
(select SId,score from study.dbo.SC where CId='')as t1,
(select SId,score from study.dbo.SC where CId='')as t2
where t1.SId=t2.SId and t1.score>t2.score) as r
on Student.SId=r.学生ID

 

join -- on这个也是常用的思路,当要连接两个某一列相关的表时。

 

1.1查询存在" 01 "课程但可能不存在" 02 "课程的情况(不存在时显示为 null )

和第一题思路类似,注意以01课程为准,所以要用left join

select *
from    
(select SId,score from study.dbo.SC where CId='') as t1
left join
(select SId,score from study.dbo.SC where CId='') as t2
on t1.SId=t2.SId

 


1.2 查询同时存在01和02课程的情况

很简单,用inner join,求两表交集

select t1.SId,t1.score,t2.score
from    
(select SId,score from study.dbo.SC where CId='') as t1
inner join
(select SId,score from study.dbo.SC where CId='') as t2
on t1.SId=t2.SId


1.3查询选择了02课程但没有01课程的情况

我的思路是还是用一个right join,然后判断NULL值,不知道会不会比not in效率高

select t2.SId,t2.score
from    
(select SId,score from study.dbo.SC where CId='') as t1
right join
(select SId,score from study.dbo.SC where CId='') as t2
on t1.SId=t2.SId
where t1.score is null

 

2.查询平均成绩大于等于 60 分的同学的学生编号和学生姓名和平均成绩

肯定要连表,有表一有表四。平均成绩涉及到group by,对平均成绩的限制涉及到having语句

select  t1.SId,t1.avg_score,t2.Sname
from
(
select SId,AVG(score) as avg_score
from study.dbo.SC
group by SId
having AVG(score)>60
) as t1
inner join study.dbo.Student as t2
on t1.SId=t2.SId

 

3.查询在 SC 表存在成绩的学生信息
依然是连表查询,表一的sid等于表四的sid,去除重复值使用DISTINCT即可

select DISTINCT Student.SId,Student.Sname,Student.Sage,Student.Ssex
from study.dbo.SC
inner join Student
on SC.SId=Student.SId

 

4.查询所有同学的学生编号、学生姓名、选课总数、所有课程的总成绩(没成绩的显示为 null )

依然是连表查询,left join

select Student.*,t2.count_id,t2.avg_score
from
Student
left join
(select SId,count(CId) as count_id ,avg(score)as avg_score from study.dbo.SC group by SId) as t2
on Student.SId=t2.SId

 

4.1 查有成绩的学生信息

inner join,不赘述

select Student.*,t2.count_id,t2.avg_score
from
Student
inner join
(select SId,count(CId) as count_id ,avg(score)as avg_score from study.dbo.SC group by SId) as t2
on Student.SId=t2.SId

 

5.查询「李」姓老师的数量

最简单的一题,知道like这种模糊查询就行

select COUNT(*)
from Teacher
where Tname like '李%'

 

6.查询学过「张三」老师授课的同学的信息

这个有意思,代表着从一张表跳到另一张表找信息

第一个思路当然是用join,多个表一个一个on连接起来

select Student.*
from
(select tid from Teacher where Tname='张三') as t1
inner join Course on t1.tid=Course.TId
inner join SC on Course.CId=SC.CId
inner join Student on SC.SId=Student.SId

但是也有另一种写法

select study.dbo.Student.*
from teacher,study.dbo.Course  ,study.dbo.student,study.dbo.sc
where teacher.Tname='张三' and   teacher.TId=Course.TId and   Course.CId=sc.CId and   sc.SId=student.SId

直接from多个表,在where里写=

我查了一下,其实这种方式是用了隐式的inner join,效率差异不大

 

 

 


7.查询没有学全所有课程的同学的信息

查到没有学全所有课程同学的sid很简单,在表4中查询。同学的信息用inner join联表1查询实现。

SELECT *
FROM study.dbo.Student as t1
inner join
(select Student.SId from Student
left join
study.dbo.SC
on Student.SId=SC.SId
group by Student.SId
having COUNT(SC.CId)!=(select count(*) from study.dbo.Course)) as t2
on t1.SId=t2.SId

 

版权声明
本文为[将来的事]所创,转载请带上原文链接,感谢
https://www.cnblogs.com/take-it-easy/p/11775991.html