SQLZOO:Using Null

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本文深入探讨了SQL查询的高级技巧，包括使用NULL、JOIN、COALESCE、COUNT、CASE语句进行复杂数据处理的方法，以及如何确保所有部门和教师被正确列出，即使他们没有直接关联。

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

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.

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.

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.

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’

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.

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.

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.

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

9.select name,case when dept in (1,2) then ‘Sci’ else ‘Art’ end
from teacher;

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

10.select name,case when dept in (1,2) then ‘Sci’ else ‘Art’ end
from teacher;

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