Find the total number of students.
select count(distinct id) from student
Now, find the number by department.
select count(distinct t2.id), t1.dept_name from department as t1 join student as t2 on t1.dept_name = t2.dept_name group by t1.dept_name
Also, find the number of instructors in each department.
select count(distinct t2.id), count(distinct t3.id), t3.dept_name from department as t1 join student as t2 on t1.dept_name = t2.dept_name join instructor as t3 on t1.dept_name = t3.dept_name group by t3.dept_name