PL/SQL ROWNUM

ROWNUM is a pseudocolumn (not a real column) that is available in a query. ROWNUM will be assigned the numbers 1, 2, 3, 4, … N , where N is the number of rows in the set ROWNUM is used with. A ROWNUM value is not assigned permanently to a row. A row in a table does not have a number; you cannot ask for row 5 from a table—there is no such thing.

ROWNUM value is actually assigned. A ROWNUM value is assigned to a row after it passes the predicate phase of the query but before the query does any sorting or aggregation. Also, a ROWNUM value is incremented only after it is assigned, which is why the following query will never return a row:

select * from t 
where ROWNUM > 1;

Below query will give you the wrong results,

select *   from emp 
where ROWNUM <= 5 
order by sal desc;

To understand the above, check the below query with this structure:

select ..., ROWNUM
from t
where <where clause>
group by <columns>
having <having clause>
order by <columns>;

Think of it as being processed in this order:

• The FROM/WHERE clause goes first.
• ROWNUM is assigned and incremented to each output row from the FROM/WHERE clause.
• SELECT is applied.
• GROUP BY is applied.
• HAVING is applied.
• ORDER BY is applied.

Below is the way ti use the rownum

select *   from  
( select * from emp 
order by sal desc ) 
where ROWNUM <= 5;

References

Oracle Blog