First n rows

Question

Version: Oracle 10
How can I get first 'n' rows in a select query but after applying order by;
For eg:
--Get first 4 rows?
select * from abc order by a;
As I understand rownum could be used in "where" clause which means it happens before order by is applied.

Answer 1

I have some examples for emp for top-N and bottom-N Queries
(just in case you want to extend you sql)
TABLE employees
Name             Null?    Type        
EMPLOYEE_ID      NOT NULL NUMBER(6)   
FIRST_NAME                VARCHAR2(20)
LAST_NAME        NOT NULL VARCHAR2(25)
EMAIL            NOT NULL VARCHAR2(25)
PHONE_NUMBER              VARCHAR2(20)
HIRE_DATE        NOT NULL DATE        
JOB_ID           NOT NULL VARCHAR2(10)
SALARY                    NUMBER(8,2) 
COMMISSION_PCT            NUMBER(2,2) 
MANAGER_ID                NUMBER(6)   
DEPARTMENT_ID             NUMBER(4)  
Task:
Display top two salaries in each department.
1. Using function ROW_NUMBER ():
SELECT employee_id, department_id, salary
  FROM (SELECT employee_id, department_id, salary,
               ROW_NUMBER () OVER (PARTITION BY department_id ORDER BY salary DESC)
                                                                           r_no
          FROM employees)
WHERE r_no <= 2
EMPLOYEE_ID DEPARTMENT_ID SALARY  
200         10            4400    
201         20            13000   
202         20            6000    
114         30            11000   
115         30            3100    
203         40            6500    
100         90            24000   
101         90            17000   
2. Using function RANK()
SELECT employee_id, department_id, salary, rn
  FROM (SELECT employee_id, department_id, salary,
               RANK () OVER (PARTITION BY department_id ORDER BY salary DESC)
                                                                           rn
          FROM employees)
WHERE rn <= 2
EMPLOYEE_ID DEPARTMENT_ID SALARY   RN                                    
200         10            4400     1                                     
201         20            13000    1                                     
202         20            6000     2                                     
114         30            11000    1                                     
115         30            3100     2                                     
203         40            6500     1                                     
100         90            24000    1                                     
101         90            17000    2                                     
102         90            17000    2                                     
The salaries that are equal (department 90) receive the same rank