SQLZOO刷题之Using Null_use count to show the number of teachers and the n-优快云博客

本文链接：https://blog.youkuaiyun.com/qq_37587948/article/details/106386294

这篇博客介绍了SQLZOO中的Using Null题目，涉及了如何使用NULL、INNER JOIN、LEFT JOIN、RIGHT JOIN来处理教师和部门的数据。内容包括列出无部门的教师、显示所有教师或部门、使用COALESCE填充缺失的移动电话号码、用CASE表达式区分教师所属类别等。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

Using Null
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
…
Teachers and Departments

The school includes many departments. Most teachers work exclusively for a single department. Some teachers have no department.

Summary
NULL, INNER JOIN, LEFT JOIN, RIGHT JOIN

List the teachers who have NULL for their department.

select name
from teacher
where dept is null;

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

SELECT teacher.name, dept.name
 FROM teacher INNER JOIN dept
           ON (teacher.dept=dept.id)

Use a different JOIN so that all teachers are listed.

SELECT teacher.name, dept.name
 FROM teacher left JOIN dept
           ON (teacher.dept=dept.id)

Use a different JOIN so that all departments are listed.

select teacher.name, dept.name
 from dept left join teacher
    on (teacher.dept=dept.id)

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’

SELECT name, COALESCE(mobile, '07986 444 2266')
FROM teacher

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.

SELECT teacher.name, COALESCE(dept.name, 'None')
 FROM teacher left JOIN dept
           ON (teacher.dept=dept.id)

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

select count(name), count(mobile)
from teacher

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.

select D.name, count(T.name)
from  teacher T right join dept D 
on T.dept = D.id
group by D.name;

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

select name, 
  case when dept in(1,2) 
    then 'Sci' 
    else 'Art' 
  end
from teacher;

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.

select name, 
  case when dept in(1,2) 
    then 'Sci' 
    when dept=3 
    then 'Art'
    else 'None' 
  end
from teacher;