Comparing value to that of previous row depending on different field - sql

Question

I have SQL which outputs rows of a date time stamp and a status change flag (0 or 1)
I need to get the timespan from the first record, where the flag will be 0, to when the status flag changes to 1, ignore records when the flag is still at 1, then get the time span after it changes back to 0 till the last record. The status change flag may flip between 0 and 1 any number of times.
So i need to be able to compare the status change flag to the previous row, and decide whether I need to keep accumulating the difference in the date time stamps.
I have been looking into writing a cursor but keep reading about how cursors are horribly inefficient.
Hopes this make any sense.

Answer 1

DECLARE #test TABLE ([group] int,t DateTime,[status] bit)
INSERT INTO #test values (1,'20130101 11:11:11',0)
INSERT INTO #test values (1,'20130101 11:11:12',0)
INSERT INTO #test values (1,'20130101 11:11:13',0)
INSERT INTO #test values (1,'20130101 11:11:14',1)
INSERT INTO #test values (1,'20130101 11:11:15',1)
INSERT INTO #test values (1,'20130101 11:11:16',1)
INSERT INTO #test values (1,'20130101 11:11:17',0)
INSERT INTO #test values (1,'20130101 11:11:18',0)
INSERT INTO #test values (1,'20130101 11:11:19',0)
Select [Group],MIN(t)
,(Select MAX(t) from #test t2 where [status]=0 and t2.[group]=t.[group] and Exists(Select * from #test t3 where [status]=1 and t3.[group]=t.[group] and t3.t<t2.t))
,DateDiff(ss,MIN(t)
,(Select MAX(t) from #test t2 where [status]=0 and t2.[group]=t.[group] and Exists(Select * from #test t3 where [status]=1 and t3.[group]=t.[group] and t3.t<t2.t))
) as Seconds
from #test t where Status=0
group by [group]

Answer 2

I think something like this will work. But I might need more info on the table structure
WITH FirstFlag(FlagType, FlagTime)
AS
(
SELECT
FlagType
, min(DateCreated) as FlagTime
FROM TheTable
WHERE Flag = 0
)
, SecondFlag(FlagTime1, FlagTime2)
AS
(
SELECT
F.FlagTime as FlagTime
, min(T.DateCreated) as FlagTime
FROM TheTable as T
INNER JOIN FirstFlag as F
ON T.FlagType = F.FlagType
WHERE Flag = 1
AND T.DateCreated > F.FlagTime
)
SELECT datediff(min, FlagTime1, FlagTime2)
FROM SecondFlag