WITH proj_prev_dates AS 
(
    SELECT Task_ID, Start_Date, End_Date, 
        LAG(End_Date) OVER(ORDER BY Start_Date) AS prev_date
    FROM Projects
    ),
proj_flags AS
(
    SELECT 
        ppd.*,
        CASE 
            WHEN prev_date IS NULL THEN 1
            WHEN prev_date = Start_Date THEN 0 
            ELSE 1
        END as proj_flag  
    FROM proj_prev_dates ppd
    ),
proj_labelled AS 
(
    SELECT pf.*,
        SUM(proj_flag) OVER(ORDER BY Start_Date) AS proj_ID
    FROM proj_flags pf  
    )
SELECT min_date, max_date
FROM
(   SELECT 
        proj_ID, 
        MIN(Start_Date) AS min_date, 
        MAX(End_Date) AS max_date,
        MAX(End_Date) - MIN(Start_Date) AS  time_taken
    FROM proj_labelled
    GROUP BY proj_ID
    )
ORDER BY time_taken, min_date;