The ROWNUM returns a number representing the order in which a row was selected from the table. The first row selected has a ROWNUM of 1; the second row has a ROWNUM of 2, and so on. If the SELECT statement involves an ORDER BY clause, then the ROWNUMs are assigned to the retrieved rows before the sort is complete.
You can use the ROWNUM in an UPDATE statement to assign exclusive values to each row in a table. You can also use ROWNUM in the WHERE clause of a SELECT statement to the limit the number of rows retrieved, as shown:
CURSOR c1 IS SELECT empno, sal FROM emp
WHERE sal > 2000 AND ROWNUM < 10; -- returns 10 rows
The value of ROWNUM increase only whenever a row is retrieved, so the only significant use of ROWNUM in a WHERE clause is
... WHERE ROWNUM < constant;