WITH difference AS (
    SELECT 
        start_date, 
        end_date,
        @group := IF(@prev_end = start_date, @group, @group + 1) AS grp,
        @prev_end := end_date
    FROM projects, (SELECT @prev_end := NULL, @group := 0) vars
    ORDER BY 1
)
SELECT 
    MIN(start_date) AS start_date, 
    MAX(end_date) AS end_date
FROM difference
GROUP BY grp
ORDER BY count(grp), 1;