WITH CTE AS 
(
SELECT
START_DATE
,END_DATE
,CASE 
WHEN ROW_NUMBER() OVER(ORDER BY END_DATE DESC) = 1  THEN END_DATE  
WHEN LEAD(END_DATE) OVER(ORDER BY END_DATE ASC)- END_DATE>1 THEN END_DATE ELSE NULL END AS PROJECT_END_DATE
,CASE WHEN ROW_NUMBER() OVER(ORDER BY END_DATE ASC) = 1 THEN START_DATE 
WHEN START_DATE - LAG(START_DATE) OVER(ORDER BY END_DATE ASC)>1 THEN START_DATE
ELSE NULL END AS PROJECT_START_DATE
FROM PROJECTS
)
,SUBQ AS 
(
SELECT
START_DATE
,END_DATE
,PROJECT_START_DATE
,PROJECT_END_DATE
FROM CTE
)
,CALC AS
(
SELECT 
STRT.PROJECT_START_DATE
,END.PROJECT_END_DATE
,1+END.PROJECT_END_DATE - STRT.PROJECT_START_DATE AS PROJECT_LENGTH
FROM 
(
SELECT 
x.PROJECT_START_DATE
,ROW_NUMBER() OVER(ORDER BY PROJECT_START_DATE ASC) AS RN__
FROM (SELECT * FROM SUBQ WHERE  PROJECT_START_DATE IS NOT NULL) x
)STRT
INNER JOIN 
(
SELECT 
y.PROJECT_END_DATE
,ROW_NUMBER() OVER(ORDER BY PROJECT_END_DATE ASC) AS RN__
FROM (SELECT * FROM SUBQ WHERE  PROJECT_END_DATE IS NOT NULL) y
)END
ON STRT.RN__ = END.RN__
)
SELECT
PROJECT_START_DATE, PROJECT_END_DATE
FROM CALC
ORDER BY PROJECT_LENGTH ASC, PROJECT_START_DATE ASC;