sqlzoo练习答案--Using Null

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最新推荐文章于 2025-01-28 02:00:00 发布

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分类专栏：基础--2.数据库（Mysql）文章标签： sqlzoo sql练习 sql

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本文通过具体实例介绍了使用SQL进行数据查询的各种技巧，包括如何处理空值、不同类型的JOIN操作、使用COALESCE函数处理缺失信息、利用COUNT进行计数汇总及分组、运用CASE表达式实现条件判断等。

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teacher
id	dept	name	phone	mobile
101	1	Shrivell	2753	07986 555 1234
102	1	Throd	2754	07122 555 1920
103	1	Splint	2293
104		Spiregrain	3287
105	2	Cutflower	3212	07996 555 6574
106		Deadyawn	3345
...

dept
id	name
1	Computing
2	Design
3	Engineering
...

1、

List the teachers who have NULL for their department.

Why we cannot use =

You might think that the phrase dept=NULL would work here but it doesn't - you can use the phrase dept IS NULL

That's not a proper explanation.

No it's not, but you can read a better explanation at Wikipedia:NULL

[sql]view plaincopy 
     
 select name from teacher where dept IS NULL  

2、 Note the INNER JOIN misses the teachers with no department and the departments with no teacher.

[sql]view plaincopy 
     
 SELECT teacher.name, dept.name  
  FROM teacher INNER JOIN dept  
            ON (teacher.dept=dept.id)  

3、 Use a different JOIN so that all teachers are listed.

[sql]view plaincopy 
     
 SELECT teacher.name, dept.name  
  FROM teacher left JOIN dept  
            ON (teacher.dept=dept.id)  

4、 Use a different JOIN so that all departments are listed

[sql]view plaincopy 
     
 SELECT teacher.name, dept.name  
  FROM teacher right JOIN dept  
            ON (teacher.dept=dept.id)  

5、 Use COALESCE to print the mobile number. Use the number '07986 444 2266' if there is no number given. Show teacher name and mobile number or '07986 444 2266'

[sql]view plaincopy 
     
 select name,COALESCE(mobile,'07986 444 2266') from teacher  

6、 Use the COALESCE function and a LEFT JOIN to print the teacher name and department name. Use the string 'None' where there is no department.

[sql]view plaincopy 
     
 SELECT teacher.name, COALESCE(dept.name,'None')  
  FROM teacher left JOIN dept  
            ON (teacher.dept=dept.id)  

7、 Use COUNT to show the number of teachers and the number of mobile phones.

[sql]view plaincopy 
     
 SELECT count(name),count(mobile) FROM teacher  

8、 Use COUNT and GROUP BY dept.name to show each department and the number of staff. Use a RIGHT JOIN to ensure that the Engineering department is listed.

[sql]view plaincopy 
     
 select dept.name ,count(teacher.name) from teacher right join dept on teacher.dept=dept.id group by dept.name   

9、 Use CASE to show the name of each teacher followed by 'Sci' if the teacher is in dept 1 or 2 and 'Art' otherwise.

[sql]view plaincopy 
     
 select name,  
 case when dept in(1,2) then 'Sci'  
 else 'Art' end  
 from teacher  

10、 Use CASE to show the name of each teacher followed by 'Sci' if the teacher is in dept 1 or 2, show 'Art' if the teacher's dept is 3 and 'None' otherwise.

[sql]view plaincopy 
     
 select name,  
 case when dept in(1,2) then 'Sci'  
 when dept=3 then 'Art'  
 else 'None' end  
 from teacher  

原文链接 http://blog.youkuaiyun.com/crazy__chen/article/details/50517891