count-student-numbers-in-departments.sql

/*

Problem

A university uses 2 data tables, student and department, to store data about its students and the departments associated with each major.

Write a query to print the respective department name and number of students majoring in each department for all departments in the department table (even ones with no current students).

Sort your results by descending number of students; if two or more departments have the same number of students, then sort those departments alphabetically by department name.

The student is described as follow:

Column Name	Type
student_id	Integer
student_name	String
gender	Character
dept_id	Integer
where student_id is the student’s ID number, student_name is the student’s name, gender is their gender, and dept_id is the department ID associated with their declared major.

And the department table is described as below:

Column Name	Type
dept_id	Integer
dept_name	String
where dept_id is the department’s ID number and dept_name is the department name.

Here is an example input:
student table:

student_id	student_name	gender	dept_id
1	Jack	M	1
2	Jane	F	1
3	Mark	M	2
department table:

dept_id	dept_name
1	Engineering
2	Science
3	Law
The Output should be:

dept_name	student_number
Engineering	2
Science	1
Law	0

*/

# V0
select 
    a.dept_name,
    coalesce(count(student_id), 0) student_number
from 
    department a 
left join
    student b
on 
    a.dept_id = b.dept_id
group by a.dept_name
order by student_number desc, a.dept_name asc;

# V1
# https://www.datageekinme.com/general/leetcode/leetcode-sql-580-count-student-number-in-departments/
select 
    a.dept_name,
    coalesce(count(student_id), 0) student_number
from 
    department a 
left join
    student b
on 
    (a.dept_id = b.dept_id)
group by a.dept_name
order by student_number desc, a.dept_name asc;

# V1'
# https://nifannn.github.io/2017/10/27/SQL-Notes-Leetcode-580-Count-Student-Number-in-Departments/
SELECT dept_name, COUNT(student_id) AS student_number FROM
department AS d LEFT JOIN student AS s ON d.dept_id = s.dept_id
GROUP BY dept_name
ORDER BY student_number DESC, dept_name;

# V2