WITH T (col1,col2,col3)AS (
   SELECT chr(64+level) , level , TO_CHAR(SYSDATE + level * 5 ,'YYYYMMDD') 
   FROM DUAL CONNECT BY LEVEL <= 10
)

-- 이번달만 조건 걸고 싶습니다.
SELECT SUM(col2) FROM T
-- 1.번 WHERE col3 Like '201305' || '%'
-- 2.번 WHERE col3 BETWEEN '201305'||'01' AND TO_CHAR(LAST_DAY(TO_DATE('201305','YYYYMM')),'YYYYMMDD')
-- 3.번 WHERE col3 BETWEEN '201305'||'01' AND '201305'||'99'