Frequently Asked interview question on queries in Oracle

Below queries are asked in the interviews frequently

1. To find the nth row of a table

SQL> Select *from employee where rowid = (select max(rowid) from employee where rownum 
        <= 4);
                              Or

SQL> Select *from employee where rownum <= 4 minus select *from employee where rownum 
           <= 3;

2. To find duplicate rows

SQL> Select *from employee where rowid in (select max(rowid) from employee group by 
         empno, ename, mgr, job, hiredate, comm, deptno, sal);
  							Or
 SQL> Select empno,ename,sal,job,hiredate,comm , count(*) from employee group by 
         empno,ename,sal,job,hiredate,comm  having count(*) >=1; 

3. To delete duplicate rows

   SQL> Delete from employee where rowid in (select max(rowid) from employee group by
   empno,ename,mgr,job,hiredate,sal,comm,deptno);

4. To find the count of duplicate rows

   SQL> Select ename, count() from employee group by ename having count() >= 1;

5. How to display alternative rows in a table?

     SQL> select *from employee where (rowid,0) in (select rowid,mod(rownum,2) from employee );
   

6. Getting employee details of each department who is drawing maximum sal?

   SQL> select *from employee where (deptno,sal) in
   ( select deptno,max(sal) from employee 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 employee group by deptno having count() >2500;

8. 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;
   

9. To find nth maximum sal

  SQL> Select *from employee where sal in (select max(sal) from (select *from emp order 
          by sal) where rownum <= 5);