SELECTing top N rows without ROWNUM?

Since this is homework, a hint rather than an answer. You'll want to use analytic functions. ROW_NUMBER, RANK, or DENSE_RANK can work depending on how you want to handle ties.

If analytic functions are also disallowed, the other option I could imagine-- one that you would never, ever, ever actually write in practice, would be something like

SELECT name, salary  FROM staff s1 WHERE (SELECT COUNT(*)          FROM staff s2         WHERE s1.salary < s2.salary) <= 3

With regard to performance, I wouldn't rely on the COST number from the query plan-- that's only an estimate and it is not generally possible to compare the cost between plans for different SQL statements. You're much better off looking at something like the number of consistent gets the query actually does and considering how the query performance will scale as the number of rows in the table increases. The third option is going to be radically less efficient than the other two simply because it needs to scan the STAFF table twice.

I don't have your STAFF table, so I'll use the EMP table from the SCOTT schema

The analytic function solution actually does 7 consistent gets as does the ROWNUM solution

Wrote file afiedt.buf  1  select ename, sal  2    from( select ename,  3                 sal,  4                 rank() over (order by sal) rnk  5            from emp )  6*  where rnk <= 3SQL> /ENAME             SAL---------- ----------smith             800SM0               950ADAMS            1110Execution Plan----------------------------------------------------------Plan hash value: 3291446077---------------------------------------------------------------------------------| Id  | Operation                | Name | Rows  | Bytes | Cost (%CPU)| Time|---------------------------------------------------------------------------------|   0 | SELECT STATEMENT         |      |    14 |   672 |     4  (25)| 00:00:01|*  1 |  VIEW                    |      |    14 |   672 |     4  (25)| 00:00:01|*  2 |   WINDOW SORT PUSHED RANK|      |    14 |   140 |     4  (25)| 00:00:01|   3 |    TABLE ACCESS FULL     | EMP  |    14 |   140 |     3   (0)| 00:00:01---------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   1 - filter("RNK"<=3)   2 - filter(RANK() OVER ( ORDER BY "SAL")<=3)Statistics----------------------------------------------------------          0  recursive calls          0  db block gets          7  consistent gets          0  physical reads          0  redo size        668  bytes sent via SQL*Net to client        524  bytes received via SQL*Net from client          2  SQL*Net roundtrips to/from client          1  sorts (memory)          0  sorts (disk)          3  rows processedSQL> select ename, sal  2    from( select ename, sal  3            from emp  4           order by sal )  5   where rownum <= 3;ENAME             SAL---------- ----------smith             800SM0               950ADAMS            1110Execution Plan----------------------------------------------------------Plan hash value: 1744961472--------------------------------------------------------------------------------| Id  | Operation               | Name | Rows  | Bytes | Cost (%CPU)| Time     |--------------------------------------------------------------------------------|   0 | SELECT STATEMENT        |      |     3 |   105 |     4  (25)| 00:00:01 ||*  1 |  COUNT STOPKEY          |      |       |       |            |          ||   2 |   VIEW                  |      |    14 |   490 |     4  (25)| 00:00:01 ||*  3 |    SORT ORDER BY STOPKEY|      |    14 |   140 |     4  (25)| 00:00:01 ||   4 |     TABLE ACCESS FULL   | EMP  |    14 |   140 |     3   (0)| 00:00:01 |--------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   1 - filter(ROWNUM<=3)   3 - filter(ROWNUM<=3)Statistics----------------------------------------------------------          1  recursive calls          0  db block gets          7  consistent gets          0  physical reads          0  redo size        668  bytes sent via SQL*Net to client        524  bytes received via SQL*Net from client          2  SQL*Net roundtrips to/from client          1  sorts (memory)          0  sorts (disk)          3  rows processed

The COUNT(*) solution, however, actually does 99 consistent gets and has to do a full scan of the table twice so it is more than 10 times less efficient. And it will scale much worse as the number of rows in the table increases

SQL> select ename, sal  2    from emp e1  3   where (select count(*) from emp e2 where e1.sal < e2.sal) <= 3;ENAME             SAL---------- ----------JONES            2975SCOTT            3000KING             5000FORD             3000FOOExecution Plan----------------------------------------------------------Plan hash value: 2649664444----------------------------------------------------------------------------| Id  | Operation           | Name | Rows  | Bytes | Cost (%CPU)| Time     |----------------------------------------------------------------------------|   0 | SELECT STATEMENT    |      |    14 |   140 |    24   (0)| 00:00:01 ||*  1 |  FILTER             |      |       |       |            |          ||   2 |   TABLE ACCESS FULL | EMP  |    14 |   140 |     3   (0)| 00:00:01 ||   3 |   SORT AGGREGATE    |      |     1 |     4 |            |          ||*  4 |    TABLE ACCESS FULL| EMP  |     1 |     4 |     3   (0)| 00:00:01 |----------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   1 - filter( (SELECT COUNT(*) FROM "EMP" "E2" WHERE              "E2"."SAL">:B1)<=3)   4 - filter("E2"."SAL">:B1)Statistics----------------------------------------------------------          0  recursive calls          0  db block gets         99  consistent gets          0  physical reads          0  redo size        691  bytes sent via SQL*Net to client        524  bytes received via SQL*Net from client          2  SQL*Net roundtrips to/from client          0  sorts (memory)          0  sorts (disk)          5  rows processed

The reason you must wrap the statement with another select is because the outer select statement is the one that limits your result set to the row numbers desired. Here's a helpful link on analytics. If you run the inner select by itself you'll see why you have to do this. Analytics are applied AFTER the where clause is evaluated, which is why you get the error that myorder is an invalid identifier.

Oracle? What about window functions?

select * from (SELECT s.*, row_number over (order by salary desc ) as rn FROM staff s )where rn <=3