sqlzoo第八关Using Null

第八关 Using Null

• 老师和部门数据库

表1 老师表（teacher）

id(编号)	dept(部门)	name（名字）	phone（电话）	mobile（手机）
101	1	Shrivell	2753	07986 555 1234
102	1	Throd	2754	07122 555 1920
103	1	Splint	2293
…	…	…	…	…

表2 部门表（dept）

id(编号)	name（名字）
1	Computing
2	Design
3	Engineering
…	…

8.1题目及答案

1. List the teachers who have NULL for their department.

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 t.name,coalesce(dept.name,'None') from teacher t
left join dept on (t.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 d.name,count(t.name) c from teacher t
	right join dept d on (t.dept=d.id)
	group by t.dept,d.name order by c desc

8.2总结

函数	用法说明
inner join	如果表中有至少一个匹配，则返回行(两表交集)
left join	即使右表中没有匹配，也从左表返回所有的行（左边全部）
right join	即使左表中没有匹配，也从右表返回所有的行（右表全部）

来源：CSDN

作者：smileKutper

链接：https://blog.csdn.net/smileKutper/article/details/103811181