Oracle SQL如何填写缺少的月份和相应的0

Question

我有问题要填写缺少的月份和相应的0

    SELECT TO_CHAR( tab.date, 'yyyy' ) "YEAR", 
            TO_NUMBER( TO_CHAR( tab.date, 'mm' ) )  "MONTH",
            SUM(CASE tab45.id WHEN '421' THEN 1 ELSE 0 END) "421"
    FROM  tab, 
          tab45,
          tab455
    WHERE ( tab.date IS NOT NULL ) AND
          ( tab.fk_id = tab45.pk_id ) AND
          ( tab.fk2_id = tab455.pk_id )
    GROUP BY  ( TO_CHAR( tab.date, 'yyyy' ) , 
                TO_NUMBER( TO_CHAR( tab.date, 'mm' ) ) )
    ORDER BY 1 asc ,2 asc

返回......

year      month        421
---- ---------- ---------- 
2014          4          3 
2014          5          5 

我怎么样......

year      month        421
---- ---------- ----------
2014          1          0
2014          2          0
2014          3          0
2014          4          3 
2014          5          5
etc.