안녕하세요. 이렇게 해 보시지요?  (오라클 기준)

WITH T AS (
SELECT '' AA, 4 BB, 5 CC FROM dual UNION ALL
SELECT 'sd',  4,    5    FROM dual UNION ALL
SELECT '',    4,    7    FROM dual UNION ALL
SELECT '',    6,    3    FROM dual 
)
SELECT aa, bb, cc 
FROM (
      SELECT aa, bb, cc, 
             (SELECT COUNT(*) 
             FROM t b 
             WHERE (a.bb = b.bb AND a.cc = b.cc) 
             ) dd
      FROM t a
     )
WHERE CASE WHEN dd = 1 THEN 'Y'
           ELSE CASE WHEN aa IS NOT NULL THEN 'Y'
                     ELSE 'N'
                END
      END = 'Y'
ORDER BY aa, bb, cc;

감사합니다.

그런데 꼭 테이블을 두번 읽는 방법밖에는 없을까요?

with tmp as (
select null as AA, 4 as BB, 5 as CC from dual union all
select 'sd' as AA, 4 as BB, 5 as CC from dual union all
select null as AA, 4 as BB, 7 as CC from dual union all
select null as AA, 6 as BB, 3 as CC from dual 
)
select AA, BB, CC
from (
      select AA, BB, CC
      , count(AA) over(partition by BB, CC) as ct
      from tmp
      ) t
where ct = 0
or AA is not null

널인 행과 아닌 행이 각각 한건씩만 존재할까요?
아니면 여러건 존재가 가능할까요?
여러건 존재한다면? 어떤 결과가 나와야 할까요?
1건씩만 가능하다면? rn 을 이용하시고
여러건 가능하다면? rk 를 이용해 보세요.

WITH t AS
(
SELECT null aa, 4 bb, 5 cc FROM dual
UNION ALL SELECT 'sd', 4, 5 FROM dual
UNION ALL SELECT 'ai', 4, 5 FROM dual
UNION ALL SELECT null, 4, 7 FROM dual
UNION ALL SELECT null, 6, 3 FROM dual
)
SELECT aa, bb, cc
  FROM (SELECT aa, bb, cc
        --   , ROW_NUMBER() OVER(PARTITION BY bb, cc ORDER BY aa) rn
             , RANK() OVER(PARTITION BY bb, cc ORDER BY NVL2(aa, 1, 2)) rk
          FROM t
        )
 WHERE rk = 1
;

BB, CC 기준으로 AA가 NULL이 아닌 경우는 1개만 존재한다면 아래처럼 간단히 가능할듯 합니다.

WITH T AS
(
SELECT null aa, 4 bb, 5 cc FROM dual
UNION ALL SELECT 'sd', 4, 5 FROM dual
UNION ALL SELECT null, 4, 7 FROM dual
UNION ALL SELECT null, 6, 3 FROM dual
)
SELECT MAX(AA) AA, BB, CC
FROM T
GROUP BY BB, CC
;