-- Oracle
SELECT 
    a.start_date,
    MIN(b.end_date) AS end_date
FROM 
    (SELECT start_date FROM projects 
     WHERE start_date NOT IN (SELECT end_date FROM projects)) a,
    (SELECT end_date FROM projects 
     WHERE end_date NOT IN (SELECT start_date FROM projects)) b
WHERE 
    a.start_date < b.end_date
GROUP BY 
    a.start_date
ORDER BY 
    min(b.end_date) - a.start_date, a.start_date;