select ename
, case Row_Number() over (partition by deptno
order by null
)
when 1
then deptno
end
from t

call count cpu elapsed disk query current rows
——- —— ——– ———- ———- ———- ———- ———-
Parse 2 166.66 286.72 0 124 0 0
Execute 2 0.00 143.36 0 0 0 0
Fetch 4 0.00 153.60 0 6 0 28
——- —— ——– ———- ———- ———- ———- ———-
total 8 166.66 583.68 0 130 0 28

Misses in library cache during parse: 2
Optimizer goal: CHOOSE
Parsing user id: 21

Rows Row Source Operation
——- —————————————————
14 WINDOW SORT
14 TABLE ACCESS FULL T

********************************************************************************
********************************************************************************

select ename
, case rn
when 1
then deptno
end deptno
from ( select ename
, deptno
, row_number() over (partition by deptno order by null) rn
from t
)

call count cpu elapsed disk query current rows
——- —— ——– ———- ———- ———- ———- ———-
Parse 2 0.01 215.04 0 5 0 0
Execute 2 0.00 0.00 0 0 0 0
Fetch 4 0.00 0.00 0 6 0 28
——- —— ——– ———- ———- ———- ———- ———-
total 8 0.01 215.04 0 11 0 28

Misses in library cache during parse: 2
Optimizer goal: CHOOSE
Parsing user id: 21

Rows Row Source Operation
——- —————————————————
14 VIEW
14 WINDOW SORT
14 TABLE ACCESS FULL T

********************************************************************************

I seems to me that the in-line view wins hands down!

You can also omit the in-line view and use the row_number() in the case-statement:

SQL> explain plan for
  2  select ename
  3       , case Row_Number() over (partition by deptno
  4                                order by null
  5                            )
  6         when 1
  7         then deptno
  8         end
  9    from t
 10  /

Explained.

SQL> @xplan

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------
--------------------------------------------------------------------
| Id  | Operation            |  Name       | Rows  | Bytes | Cost  |
--------------------------------------------------------------------
|   0 | SELECT STATEMENT     |             |    14 |   126 |     4 |
|   1 |  WINDOW SORT         |             |    14 |   126 |     4 |
|   2 |   TABLE ACCESS FULL  | T           |    14 |   126 |     1 |
--------------------------------------------------------------------

Note: cpu costing is off

Notice the Bytes, when compared to using an in-line view:

SQL> explain plan for
  2  select ename
  3  ,      case rn
  4         when 1
  5         then deptno
  6         end  deptno
  7  from   ( select ename
  8           ,      deptno
  9           ,      row_number() over (partition by deptno order by null) rn
 10           from   t
 11         )
 12  /

Explained.

SQL> @xplan

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------
--------------------------------------------------------------------
| Id  | Operation            |  Name       | Rows  | Bytes | Cost  |
--------------------------------------------------------------------
|   0 | SELECT STATEMENT     |             |    14 |   462 |     4 |
|   1 |  VIEW                |             |    14 |   462 |     4 |
|   2 |   WINDOW SORT        |             |    14 |   126 |     4 |
|   3 |    TABLE ACCESS FULL | T           |    14 |   126 |     1 |
--------------------------------------------------------------------

Note: cpu costing is off

(I run these scripts on a Oracle 9205, in case you’re interested)