SQLZOO练习-Part 8：Using Null

Part 8：Using Null

teacher

id	dept	name	phone	mobile
101	1	Shrivell	2753	07986 555 1234
102	1	Throd	2754	07122 555 1920
103	1	Splint	2293
104		Spiregrain	3287
105	2	Cutflower	3212	07996 555 6574
106		Deadyawn	3345
...

dept

id	name
1	Computing
2	Design
3	Engineering
...

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 AS Teacher, dept.name AS Department
FROM teacher INNER JOIN dept ON (teacher.dept=dept.id)
WHERE teacher.name IS NOT NULL  OR  teacher.dept IS NOT NULL

3. Use a different JOIN so that all teachers are listed.(列出所有老师的名字及所属部门)

SELECT teacher.name AS Teacher, dept.name AS Department
FROM teacher LEFT JOIN dept ON (teacher.dept=dept.id)

4. Use a different JOIN so that all departments are listed.

SELECT teacher.name AS Teacher, dept.name AS Department
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 teacher.name AS Teacher, 
COALESCE(teacher.mobile,'07986 444 2266') AS TEL
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 AS Teacher, 
COALESCE(dept.name,'None') AS  Department
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(teacher.name) AS TeacherNum, COUNT(teacher.mobile) AS MobileNum
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 (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 teacher.name AS Teacher, 
CASE WHEN teacher.dept=1 THEN 'Sci'
     WHEN teacher.dept=2 THEN 'Sci'
     ELSE 'Art' END 
AS Tpye
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 teacher.name AS Teacher, 
CASE WHEN teacher.dept=1 OR teacher.dept=2 THEN 'Sci'
     WHEN teacher.dept=3 THEN 'Art'
     ELSE 'None' END 
AS Tpye
FROM teacher