SqlZoo.net习题答案：NULL, INNER JOIN, LEFT JOIN, RIGHT JOIN

习题地址：http://sqlzoo.net/4a.htm

表结构：　　teacher(id, dept, name, phone, mobile)

　　　　　　dept(id, name)

　　　　　　teacher.dept = dept.id

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

select name from teacher where dept is null

1b.Note the INNER JOIN misses the teacher with no department and the department with no teacher.

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

1c.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)

1d.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)

2a.Use COALESCE to print the mobile number. Use the number '07986 444 2266' there is no number given.

Show teacher name and mobile number or '07986 444 2266'

select teacher.name, coalesce(teacher.mobile, '07986 444 2266')
from teacher

2b.Use the COALESCE function and a LEFT JOIN to print the 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)

2c.Use COUNT to show the number of teachers and the number of mobile phones.

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

2d.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.id)
from teacher right join dept
on (dept.id = teacher.dept)
group by dept.name

3a.Use CASE to show the name of each teacher followed by 'Sci' if the the teacher is in dept 1 or 2 and 'Art' otherwise.

select name,
case when dept = 1 or dept = 2 then 'Sci'  else 'Art' end
from teacher

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

select name, case when dept = 1 or dept = 2 then 'Sci' when dept = 3 then 'Art' else 'None' end
from teacher