select length(f3) - length(replace(f3, 'a','')) from t1

简单的，如果，要统计 good 出现的次数，改成

select (length(f3) - length(replace(f3, 'good ',''))) /  length('good')

from t1

-----------------------------------

SQL> select (length('XXgoodXXXXgoodxxx') -

  2         length(replace('XXgoodXXXXgoodxxx', 'good', ''))) / length('good') 次数

  3    from dual;

        次数

----------

         2