RE: find gaps between numbers

SQL> select * from t order by x;

         X
----------
         1
         2
         5
         6
         7
        10
        12
        13

SELECT ROWNUM FROM dual CONNECT BY LEVEL <= xxx MINUS SELECT col_of_numbers FROM your_table;

where xxx is max numbers that are in your_table table. In this case 13. result is:

ROWNUM
----------
         3
         4
         8
         9

SQL>