WITH t1 AS (
    SELECT CONNECT_BY_ROOT(start_date) s_date, end_date e_date, level lvl
    FROM Projects
    WHERE CONNECT_BY_ISLEAF = 1
    CONNECT BY NOCYCLE PRIOR end_date = start_date
),
t2 AS (
    SELECT s_date, e_date,
        ROW_NUMBER() OVER (PARTITION BY e_date ORDER BY lvl DESC) AS rn
    FROM t1
)
SELECT s_date, e_date
FROM t2
WHERE rn = 1
ORDER BY (e_date-s_date), s_date
;