Merging tables and getting the output in ascending order - mysql

Question

I have two tables as such:
student department
id department_id department_id department_name
5 5 5 Computer Science
1 4 4 Architecture
3 2 1 Mathematics
4 5 3 Chemistry
2 4 2 Physics
I wrote a query as follow and got the following results.
SELECT DEPARTMENTS.DEPT_NAME AS D, STUDENTS.DEPT_ID AS D_ID
FROM STUDENTS
INNER JOIN DEPARTMENTS
ON STUDENTS.DEPT_ID=DEPARTMENTS.DEPT_ID ;
Computer Science 5
Computer Science 5
Physics 2
Architecture 4
Architecture 4
It's fine till here but I want something like
Computer Science 2
Architecture 2
Physics 1
Chemistry 0
Mathematics 0
i.e department name , num_of students where num_of students are in decending order.
What can I add to the query?

Answer 1

I would use COUNT(*) and subquery it for the ORDER BY
SELECT * FROM (
SELECT DEPARTMENTS.DEPT_NAME, COUNT(*) AS num_ofstudents
FROM STUDENTS
LEFT JOIN DEPARTMENTS
ON STUDENTS.DEPT_ID=DEPARTMENTS.DEPT_ID
GROUP BY Departments.Dept_name
) AS a ORDER BY num_ofstudents
edit- Thanks AaronDietz for pointing this out!
You should replace the INNER JOIN with a LEFT JOIN so that the query includes the records from [Departments] that do not have any students. Also, I did not need to include the subquery.
SELECT DEPARTMENTS.DEPT_NAME, COUNT(*) AS num_ofstudents
FROM STUDENTS
LEFT JOIN DEPARTMENTS
ON STUDENTS.DEPT_ID=DEPARTMENTS.DEPT_ID
GROUP BY Departments.Dept_name
ORDER BY num_ofstudents

Answer 2

You can try grouping the departments and the id, then count.
SELECT DEPARTMENTS.DEPT_NAME AS D, COUNT(*) as NID
FROM STUDENTS
INNER JOIN DEPARTMENTS
ON STUDENTS.DEPT_ID=DEPARTMENTS.DEPT_ID
GROUP BY DEPARTMENTS.DEPT_NAME
ORDER BY NID DESC

Answer 3

I think the simplest approach is to select the departments and get the count in a subquery:
select
department_id,
department_name,
(select count(*) from student s where s.department_id = d.department_id) as student_count
from department d
order by 3 desc;
This works well, because you just want one value from the students, namely the count. If you wanted more information then you'd move the subquery to the from clause. E.g.:
select
d.department_id,
d.department_name,
colalesce(s.students, 0) as student_count,
s.ids as student_ids
from department d
left join
(
select
department_id,
count(*) as students,
group_concat(id) as ids
from student
group by department_id
) s on s.department_id = d.department_id
order by 3 desc;

Answer 4

SELECT DEPARTMENTS.DEPT_NAME AS D, count(STUDENTS.DEPT_ID) AS D_ID
FROM STUDENTS
INNER JOIN DEPARTMENTS
ON STUDENTS.DEPT_ID=DEPARTMENTS.DEPT_ID
GROUP BY D
ORDER BY D_ID DESC;

Answer 5

Grouping by department name
Other queries are mostly right but COUNT should be on student id and query should start from department instead of student.
SELECT DEPARTMENTS.DEPT_NAME,
COUNT(id) AS num_ofstudents
FROM DEPARTMENT
LEFT JOIN students ON STUDENTS.DEPT_ID = DEPARTMENTS.DEPT_ID
GROUP BY DEPARTMENTS.DEPT_NAME
ORDER BY num_ofstudents