Using Null.sql

#List the teachers who have NULL for their department.
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;

#Use a different JOIN so that all teachers are listed.
SELECT DISTINCT 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 DISTINCT teacher.name, dept.name
FROM teacher RIGHT JOIN dept ON teacher.dept = dept.id;

#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(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.id)
FROM teacher RIGHT JOIN dept ON teacher.dept = dept.id
GROUP BY dept.name;

#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;

#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;