DECLARE @t TABLE (ID int,FLAG int) 
INSERT INTO @t(ID,FLAG)
SELECT 1, 0 
UNION SELECT 2, 0 
UNION SELECT 3, 1 
UNION SELECT 4, 1 
UNION SELECT 5, 1 
UNION SELECT 6, 0 
UNION SELECT 7, 1 
UNION SELECT 8, 1 

;WITH 
 t1(ID,FLAG,N) AS(SELECT ID,FLAG,CASE WHEN FLAG <> 0 THEN ID - ROW_NUMBER()OVER(PARTITION BY FLAG ORDER BY ID) ELSE NULL END AS N FROM @t)
,t2(N,ID) AS(SELECT N, MIN(ID) AS ID FROM t1 WHERE N IS NOT NULL GROUP BY N)
 SELECT t1.ID, t1.FLAG, t2.ID AS FIRSTID
 FROM t1
 LEFT JOIN t2 ON t2.N = t1.N
 ORDER BY t1.ID

DECLARE @t TABLE (ID int,FLAG int) 
INSERT INTO @t(ID,FLAG)
SELECT 5, 0 
UNION SELECT 6, 0 
UNION SELECT 9, 1 
UNION SELECT 22, 1 
UNION SELECT 25, 1 
UNION SELECT 26, 0 
UNION SELECT 37, 1 
UNION SELECT 42, 1 

;WITH t1 (ID, FLAG, N) AS
(SELECT ID, FLAG, CASE WHEN FLAG <> 0 THEN ID - ROW_NUMBER() OVER (PARTITION BY FLAG ORDER BY ID) ELSE NULL END AS N FROM @t)
,t2(N,ID) AS (SELECT N, MIN(ID) AS ID FROM t1 WHERE N IS NOT NULL GROUP BY N)
 SELECT t1.ID, t1.FLAG, t2.ID AS FIRSTID
 FROM t1
 LEFT JOIN t2 ON t2.N = t1.N
 ORDER BY t1.ID

declare @t table (id int,flag int) 
insert into @t(id,flag)
values (5, 0), (6, 0), (9, 1), (22, 1), 
(25, 1), (26, 0), (37, 1), (42, 1); 

with cte as
(
select *, 
       case 
          when lag(flag) over(order by id) is null 
             then null -- no first
          when (flag <> lag(flag) over(order by id)) and 
               (lead(flag) over(order by id) = lag(flag) over(order by id)) -- the only one
             then null 
          when flag <> lag(flag) over(order by id) 
             then id 
          else -1
       end as val
from @t
)

select id, 
       flag, 
	   nullif( 
	   case 
	      when val IS null then NULL
	      else max(val) over(order by id)
	   end, -1) 
	   as first_el    
from cte;

WITH T AS
(
 SELECT * FROM
 (
  VALUES
  (1,0)
 ,(2,0)
 ,(3,1)
 ,(4,1)
 ,(5,1)
 ,(6,0)
 ,(7,1)
 ,(8,1)
 )T(ID,FLAG)
)
SELECT *
FROM T
WHERE NOT EXISTS
(
 SELECT *
 FROM T TT
 WHERE TT.FLAG=T.FLAG AND TT.ID<T.ID
   AND NOT EXISTS
   (
    SELECT *
	FROM T TTT
	WHERE TTT.FLAG<>TT.FLAG AND TTT.ID>TT.ID AND TTT.ID<T.ID
   )
);

declare @t table (id int,flag int) 
insert into @t(id,flag)
values (5, 0), (6, 0), (9, 1), (22, 1), 
(25, 1), (26, 0), (37, 1), (42, 1); 

with cte as
(
select *,	
       case 
          when flag <> lag(flag, 1, id) over(order by id) 
             then id 
          else -1
       end as val
from @t
)

select id, 
       flag, 
       case flag when 0 then null
                 else max(val) over(order by id)
	   end as first_el    
from cte;

create table #t (id int identity, flag int)
insert #t values (0), (0),(1), (1),(1), (0),(1), (1)

select id, flag, case when flag = 0 then null else min(id) over(partition by  flag1) end
from 
	(select id, flag, case when flag = 0 then null else sum(1 - flag) over(order by id) end flag1
	from #t) a
 order by id

 drop table #t