Please help SQL query

Question

why this works fine
SELECT * FROM (
SELECT ename,deptno,sal ,ROWNUM RN FROM emp)
WHERE MOD(RN,3)=0
while
SELECT rownum FROM employees WHERE MOD(Rownum,3)=0
wotnh works
why this happenssssssssssssss

Answer 1

ROWNUM is dynamically determined on-the-fly during the fetching of results.
So this works:
SELECT * FROM table WHERE rownum < 11
It will return the first 10 rows. The following does not work:
SELECT * FROM table WHERE rownum between 11 and 20
As ROWNUM is added as the results are fetched from the database, it can only be used as a stopkey.
To use ROWNUM as any other column in a predicate, you need to change ROWNUM from a "dynamic on-the-fly" column into a "proper" column. Just as you did with your 1st SELECT.
E.g.WITH my_results AS(
        SELECT rownum AS RNO, o.* FROM all_objects o WHERE object_type = 'TRIGGER'
SELECT
        object_id,
        object_name
FROM    my_results
WHERE   rno between 11 and 20