SQLZOO Select Basics Practices Solutions.

1.

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

								
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')
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')
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), COUNT(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 dept.name, COUNT(teacher.name)
FROM teacher
RIGHT JOIN dept
ON (dept.id = teacher.dept)
GROUP BY dept.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 teacher.name,
CASE
WHEN teacher.dept = 1 OR teacher.dept = 2 THEN 'Sci'
ELSE 'Art'
END AS dept_class
FROM teacher
LEFT JOIN dept 
ON (teacher.dept = dept.id);
								
							

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 teacher.name,
CASE
WHEN teacher.dept = 1 OR teacher.dept = 2 THEN 'Sci'
WHEN teacher.dept = 3 THEN 'Art'
ELSE 'None'
END AS dept_class
FROM teacher
LEFT JOIN dept 
ON (teacher.dept = dept.id);