SQL练习

1 List the teachers who have NULL for their department.

select name from teacher
where dept is null

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
注：coalesce（x,y) x is null then y

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

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

9 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=1 or dept=2 then ‘Sci’
else ‘Art’
end
from teacher

10 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=1 or dept=2 then ‘Sci’
when dept=3 then ‘Art’
else ‘None’
end
from teacher