달력테이블과 조인하는 방법을 참고해봤습니다.

http://www.gurubee.net/article/55635

http://www.gurubee.net/article/67721

WITH CALENDAR_DATES AS (  
  SELECT DATE'2021-02-28' + LEVEL CALENDAR_DATE   
  FROM   DUAL  
  CONNECT BY LEVEL <= 10
),
TMP AS (
SELECT SDT + (LV - 1) DT
  FROM (SELECT 1 NUM, TO_DATE('2021.03.02','YYYY.MM.DD') SDT, TO_DATE('2021.03.04','YYYY.MM.DD') EDT FROM DUAL UNION ALL
        SELECT 2, TO_DATE('2021.03.04','YYYY.MM.DD'), TO_DATE('2021.03.05','YYYY.MM.DD') FROM DUAL UNION ALL
        SELECT 3, TO_DATE('2021.03.05','YYYY.MM.DD'), TO_DATE('2021.03.06','YYYY.MM.DD') FROM DUAL)
     , (SELECT LEVEL LV
          FROM DUAL
       CONNECT BY LEVEL <= 10)
 WHERE LV <= EDT + 1 - SDT
 ORDER BY DT
)
SELECT CALENDAR_DATE, COUNT(T.DT)
FROM CALENDAR_DATES 
LEFT JOIN TMP T
ON T.DT = CALENDAR_DATE
GROUP BY CALENDAR_DATE
ORDER BY CALENDAR_DATE

pajama님 쿼리 참고해서 약간 심플버전(?) 입니다.

WITH dataTbl AS (
    SELECT 1 AS nums,  TO_DATE('2021.03.02','YYYY.MM.DD') AS sdate , TO_DATE('2021.03.04','YYYY.MM.DD') AS edate FROM dual UNION ALL
    SELECT 2 AS nums,  TO_DATE('2021.03.04','YYYY.MM.DD') AS sdate , TO_DATE('2021.03.05','YYYY.MM.DD') AS edate FROM dual UNION ALL
    SELECT 3 AS nums,  TO_DATE('2021.03.05','YYYY.MM.DD') AS sdate , TO_DATE('2021.03.06','YYYY.MM.DD') AS edate FROM dual
) 
,CALENDAR_DATES AS (  
    SELECT 
        TO_DATE('2021-02-28','YYYY.MM.DD') + LEVEL CALENDAR_DATE   
    FROM DUAL
    CONNECT BY LEVEL <= 8
)

SELECT
    A.CALENDAR_DATE AS check_date, COUNT(nums) AS date_cnt -- , MIN(B.sdate), MAX(B.edate) 
FROM CALENDAR_DATES A
    LEFT OUTER JOIN dataTbl B ON A.CALENDAR_DATE BETWEEN B.sdate AND B.edate
GROUP BY A.CALENDAR_DATE
ORDER BY A.CALENDAR_DATE

WITH t AS
(
SELECT 1 no, '2021.03.02' sdt, '2021.03.04' edt FROM dual
UNION ALL SELECT 2, '2021.03.04', '2021.03.05' FROM dual
UNION ALL SELECT 3, '2021.03.05', '2021.03.06' FROM dual
)
, calendar AS
(
SELECT TO_CHAR(dt + LEVEL - 1, 'yyyy.mm.dd') dt
  FROM (SELECT TO_DATE('202103', 'yyyymm') dt FROM dual)  -- 기준년월
 CONNECT BY LEVEL <= LAST_DAY(dt) - dt + 1
)
SELECT a.dt
     , COUNT(b.sdt) cnt
  FROM calendar a
  LEFT OUTER JOIN t b
    ON a.dt BETWEEN b.sdt AND b.edt
 GROUP BY a.dt
 ORDER BY a.dt
;

모두 답글 달아주셔서 감사합니다.

고맙단 말을 진작 드렸어야 하는데 일이바쁘다 보니, 감사하다는 말이 늦었네요.

날짜를 beteen 문으로 걸어서 count 하는 부분은 아직도 제가 이해가 잘 안되네요.

실력이 미천하다보니 어떻게 저렇게 했는데 결과가 나오는지....... ㅠㅠ

저는 캘린더 외에도 시작일과 종료일도 level과 connect by로 날짜를 일자별로 생성한뒤, count하는 방법을 썼는데, 그러다 보니 데이터가 많아지면 속도가 무지하게 느려지는 현상이 발생했습니다.

덕분에 많이 배우고 갑니다.

모두 감사드립니다.