1)   To find the nth row of a table
SQL> Select *from emp where rowid = (select max(rowid) from emp where rownum <= 4);
   SQL> Select *from emp where rownum <= 4 minus select *from emp where rownum <= 3;

2)   To find duplicate rows
SQL> Select *from emp where rowid in (select max(rowid) from emp group by empno,          
         ename, mgr, job, hiredate, comm, deptno, sal);
 SQL> Select empno,ename,sal,job,hiredate,comm , count(*) from emp group by 
         empno,ename,sal,job,hiredate,comm  having count(*) >=1; 

3)   To delete duplicate rows
      SQL> Delete emp where rowid in (select max(rowid) from emp group by 

4)   To find the count of duplicate rows
      SQL> Select ename, count(*) from emp group by ename having count(*) >= 1;

5)   How to display alternative rows in a table?
          SQL> select *from emp where (rowid,0) in (select rowid,mod(rownum,2) from emp);

6)   Getting employee details of each department who is drawing maximum sal?
       SQL> select *from emp where (deptno,sal) in
               ( select deptno,max(sal)  from emp group by deptno);

7)   How to get number of employees in each department  , in which department is having more than 2500 employees?
       SQL> Select deptno,count(*) from emp group by  deptno having count(*) >2500;

           9) To reset the time to the beginning of the day

                  SQL> Select to_char(trunc(sysdate),’dd-mon-yyyy hh:mi:ss am’) from dual;

10) To find nth maximum sal
  SQL> Select *from emp where sal in (select max(sal) from (select *from emp order by sal) 

          where rownum <= 5);