select a,count(b) c_b
from tb1
group by a

------解决方案--------------------
1楼正解！！！

------解决方案--------------------

用一个外连接即可搞定，思路：先找到要显示的a，再用a左外连接你的group语句，找到最终结果
SELECT   t2.rn, NVL (t1.acount, 0) acount
 FROM   (  SELECT   a, COUNT (a) acount
             FROM   tb1
         GROUP BY   a) t1,
        (    SELECT   ROWNUM rn
                                  FROM   DUAL
                            CONNECT BY   ROWNUM <= 3) t2
  WHERE   t1.a(+) = t2.rn