group by해서 count하면 되는거 아닌가요?

날짜 테이블이 있으면 날짜 테이블과 조인 하시면 되구요..

없다면 아래처럼... TTT 테이블이 크다면 속도는 좋지 않을듯 하네요..

 
WITH  TTT AS (
SELECT '2012-04-26' dt FROM dual UNION ALL
SELECT '2012-04-26' dt FROM dual UNION ALL
SELECT '2012-04-27' dt FROM dual UNION ALL
SELECT '2012-04-28' dt FROM dual UNION ALL
SELECT '2012-04-30' dt FROM dual UNION ALL
SELECT '2012-05-02' dt FROM dual UNION ALL
SELECT '2012-05-05' dt FROM dual
)
SELECT A.DT, NVL(CNT,0)
  FROM ( -- 해당 범위 날짜
         SELECT  to_char(to_date((SELECT MIN(dt) FROM TTT), 'yyyy-mm-dd')+LEVEL-1, 'yyyy-mm-dd') dt
          FROM  dual
         CONNECT by LEVEL < (SELECT to_date(MAX(dt),'yyyy-mm-dd') - to_date(MIN(dt),'yyyy-mm-dd') + 2 FROM TTT )
        ) A,
        (SELECT DT, COUNT(*) CNT
         FROM   TTT B
         GROUP  BY DT
        ) B
 WHERE  A.DT = B.DT(+)
ORDER BY A.DT