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
...

Teachers and Departments

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

Selecting NULL values.

Summary

NULL, INNER JOIN, LEFT JOIN, RIGHT JOIN

List the teachers who have NULL for their department.

You might think that the phrase dept=NULL would work here but it doesn't - you can use the phrase dept IS NULL

No it's not, but you can read a better explanation at Wikipedia:NULL.

SELECT name
FROM teacher
WHERE dept IS NULL

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)

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

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)

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)

Using the COALESCE function

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

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

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

SELECT COUNT(teacher.name), COUNT(mobile)
FROM teacher

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

Using CASE

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 IN (1,2) 
THEN 'Sci'
ELSE 'Art' END
FROM teacher

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 IN (1,2) 
THEN 'Sci'
WHEN dept = 3 
THEN 'Art'
ELSE 'None' END
FROM teacher

Clear your results

Using Null Quiz