For each row returned by a query, ROWNUM returns a number indicating the order in which Oracle RDBMS selects the row from a table or set of joined rows.
The general format is:
SELECT * FROM invoice WHERE ROWNUM < 10;
returns first 9 rows
SELECT * FROM invoice WHERE ROWNUM > 1;
this query returns no rows
If an ORDER BY clause follows ROWNUM in the same query, the results can vary depending on the way the rows are accessed.