第八关 Using Null

• 老师和部门数据库

表1 老师表（teacher）

表2 部门表（dept）

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