SQL Important Queries

SQL Important  QUERIES

1)   To find the nth row of a table

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

Or

   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);

Or

 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

              empno,ename,mgr,job,hiredate,sal,comm,deptno having count(*)>1);

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);