No of employees in dept

SELECT COUNT (DISTINCT A||B) FROM TABLE --distinct at a time in a table

Query to find the employees in a department having maximum number of emps

SELECT e.empno, e.ename, e.sal, e.deptno

FROM       emp         e

,               (

                                SELECT      deptno

                                ,                 RANK () OVER (ORDER BY COUNT (*)     DESC)      AS rnk

                                FROM         emp

                                GROUP BY  deptno

                )               s

WHERE     e.deptno  = s.deptno

AND         s.rnk                        = 1

;

Or

select empno, ename, sal, a.deptno

from emp a,

(select deptno from (select deptno, count(1) over (partition by (deptno)) from emp order by 2 desc) where rownum =1) b

where a.deptno = b.deptno;