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8_USING_NULL.md

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SQLZoo Solutions: Using NULL

This document contains my solutions to the SQLZoo 'Using NULL' section using MySQL syntax, along with my personal learning notes and explanations.

Problem 1

List the teachers who have NULL for their department.
Why we cannot use =
You might think that the phrase dept=NULL would work here but it doesn't - you can use the phrase dept IS NULL.

My Solution:

SELECT name
FROM teacher
WHERE dept IS NULL;

My Notes:
Use IS NULL to filter null values in SQL.
In SQL, NULL is not the same as a value of 0.

From Wikipedia:
"A null should not be confused with a value of 0. A null indicates a lack of a value, which is not the same as a zero value.
In SQL, null is a marker, not a value.
This usage is quite different from most programming languages, where a null value of a reference means it is not pointing to any object."

Problem 2

Note the INNER JOIN misses the teachers with no department and the departments with no teacher.

My Solution:

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

Problem 3

Use a different JOIN so that all teachers are listed.

My Solution:

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

My Notes:
LEFT JOIN includes all teachers, even those without a department.

Problem 4

Use a different JOIN so that all departments are listed.

My Solution:

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

My Notes:
RIGHT JOIN ensures all departments are listed, even if they have no teachers.

Problem 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'.

My Solution:

SELECT 
  teacher.name,
  COALESCE(teacher.mobile, '07986 444 2266') AS 'mobile number'
FROM teacher;

My Notes:
COALESCE() returns the first non-null value from its arguments.

Problem 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.

My Solution:

SELECT 
  teacher.name, 
  COALESCE(dept.name, 'None') AS 'department name'
FROM teacher
  LEFT JOIN dept ON (teacher.dept = dept.id);

Problem 7

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

My Solution:

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

Problem 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.

My Solution:

SELECT
  dept.name,
  COUNT(teacher.name)
FROM teacher
  RIGHT JOIN dept ON (teacher.dept = dept.id)
GROUP BY dept.name;

Problem 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.

My Solution:

SELECT
  teacher.name,
  (CASE 
    WHEN dept IN (1, 2) THEN 'Sci'
    ELSE 'Art' 
    END) AS 'Sci or Art'
FROM teacher;

My Notes:
CASE allows you to return different values under different conditions.

Problem 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.

My Solution:

SELECT
  teacher.name,
  (CASE
    WHEN dept IN (1, 2) THEN 'Sci'
    WHEN dept = 3 THEN 'Art'
    ELSE 'None'
  END) AS 'Sci, Art, or None'
FROM teacher;