Invoice number   Invoice user   days_process
1                ABC             2001
                 XYZ             2002
	         PQR             2003
2                OUT             9032
                 YER             9033

3               LJK             9044
                TER             3033				 
                TFT             3045

SQL> break on deptno
SQL> select deptno, ename, job, sal from emp order by deptno, ename;

    DEPTNO ENAME      JOB              SAL
---------- ---------- --------- ----------
        10 CLARK      MANAGER         3250
           KING       PRESIDENT       5800
           MILLER     CLERK           2100
        20 ADAMS      CLERK           1900
           FORD       ANALYST         3800
           JONES      MANAGER         3775
           SMITH      CLERK           1600
        30 ALLEN      SALESMAN        2400
           BLAKE      MANAGER         3650
           JAMES      CLERK           1750
           MARTIN     SALESMAN        2050
           TURNER     SALESMAN        2300
           WARD       SALESMAN        2050

select decode (lag(invoice_number) over (order by invoice_number,days_process),
               invoice_number,null,invoice_number) as invoice_number,invoice_user,days_process
from my_table
order by invoice_number,days_process;

SELECT
  CASE
    WHEN row_number() over (partition BY Invoice_number order by Invoice_user ASC)= 1
    THEN Invoice_number
    ELSE NULL
  END ,
  Invoice_user,
  days_process
FROM table_name
ORDER BY Invoice_number asc,Invoice_user asc;

SQL> select nullif(deptno,lag(deptno) over(order by deptno, ename, empno)) deptno,
  2         ename, job, sal
  3  from emp
  4  order by emp.deptno, ename, empno
  5  /
    DEPTNO ENAME      JOB              SAL
---------- ---------- --------- ----------
        10 CLARK      MANAGER         2450
           KING       PRESIDENT       5000
           MILLER     CLERK           1300
        20 ADAMS      CLERK           1100
           FORD       ANALYST         3000
           JONES      MANAGER         2975
           SCOTT      ANALYST         3000
           SMITH      CLERK            800
        30 ALLEN      SALESMAN        1600
           BLAKE      MANAGER         2850
           JAMES      CLERK            950
           MARTIN     SALESMAN        1250
           TURNER     SALESMAN        1500
           WARD       SALESMAN        1250

SQL> select decode(rank() over(partition by deptno order by ename, empno), 1,deptno) deptno,
  2         ename, job, sal
  3  from emp
  4  order by emp.deptno, ename, empno
  5  /
    DEPTNO ENAME      JOB              SAL
---------- ---------- --------- ----------
        10 CLARK      MANAGER         2450
           KING       PRESIDENT       5000
           MILLER     CLERK           1300
        20 ADAMS      CLERK           1100
           FORD       ANALYST         3000
           JONES      MANAGER         2975
           SCOTT      ANALYST         3000
           SMITH      CLERK            800
        30 ALLEN      SALESMAN        1600
           BLAKE      MANAGER         2850
           JAMES      CLERK            950
           MARTIN     SALESMAN        1250
           TURNER     SALESMAN        1500
           WARD       SALESMAN        1250