8.1 Using Null (Outer joins).md

Using Null

Link to the page - https://sqlzoo.net/w/index.php/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.

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)

name	name
Shrivell	Computing
Throd	Computing
Splint	Computing
Cutflower	Design

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)

name	name
Shrivell	Computing
Throd	Computing
Splint	Computing
Spiregrain
Cutflower	Design
Deadyawn

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)

name	name
Shrivell	Computing
Throd	Computing
Splint	Computing
Cutflower	Design
	Engineering

5. 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') as mobile
from teacher

name	COALESCE(mobi..
Shrivell	07986 555 1234
Throd	07122 555 1920
Splint	07986 444 2266
Spiregrain	07986 444 2266
Cutflower	07996 555 6574
Deadyawn	07986 444 2266

• NOTE COALESCE takes any number of arguments and returns the first value that is not null.

    COALESCE(x,y,z) = x if x is not NULL
    COALESCE(x,y,z) = y if x is NULL and y is not NULL
    COALESCE(x,y,z) = z if x and y are NULL but z is not NULL
    COALESCE(x,y,z) = NULL if x and y and z are all NULL
  

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 t.name, COALESCE(d.name, 'None') department
  from teacher t
    left join dept d ON dept = d.id

name	department
Shrivell	Computing
Throd	Computing
Splint	Computing
Spiregrain	None
Cutflower	Design
Deadyawn	None

7.

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

select count(id) "teachers total"
  , count(mobile) "mobiles total"
  from teacher

teachers total	mobiles total
6	3

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.id) "number of staff"
  from teacher t right join dept d
    on dept = d.id
  group by d.name

name	number of staff
Computing	3
Design	1
Engineering	0

9. 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 = 1 OR dept = 2) THEN
           'Sci'
         ELSE
           'Art'
    END
  from teacher
-- NOTE condition bellow should be used, but this "isnt correct"
-- where dept IS NOT NULL

name	CASE WHEN (de..
Shrivell	Sci
Throd	Sci
Splint	Sci
Spiregrain	Art
Cutflower	Sci
Deadyawn	Art

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

name	CASE WHEN (de..
Shrivell	Sci
Throd	Sci
Splint	Sci
Spiregrain	None
Cutflower	Sci
Deadyawn	None