Teacher - Dept Tables

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
...			null

dept

d	name
1	Computing
2	Design
3	Engineering
...

Teachers and Departments

The school includes many departments. Most teachers work exclusively for a single department. Some teachers have no department.

NULL, INNER JOIN, LEFT JOIN, RIGHT JOIN

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') AS mobile
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, COALESCE(dept.name, 'None') AS dept
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) AS num_teacher, COUNT(mobile) AS num_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) AS num_teacher
FROM teacher t
RIGHT JOIN dept d 
ON t.dept = d.id
GROUP BY d.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 AS new_name
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