Three record with maximum values

Question

I have followed query:
select a.deptno a, count(b.empno) b
from dept a, emp b
where b.deptno(+) = a.deptno
group by a.deptno
and I got a result:
a b
10 3
20 5
30 6
40 0
50 9
60 4
70 7
80 1
How can I get three departments with largest number of employees?
I would like to get result like this:
a b
50 9
70 7
30 6
Thanks.

Answer 1

If you have several departments with the same count of employees then a limitation by rownum leads to a undefined result,
e.g. when department 20 in your eample has also 6 employees, then the answer could be
a b
50 9
70 7
30 6
20 6
use the dense_rank function to get the rank of the departments and then limit to the top three,
e.g.
select *
from (
select a, b, dense_rank() over(order by b desc) b_rank
from (
select a.deptno a, count(b.empno) b
from dept a, emp b
where b.deptno(+) = a.deptno
group by a.deptno
where b_rank < 4
;