Capture las horas de inicio y finalización de los cambios de estado de un campo

Si tiene SQL Server 2012 o posterior, puede usar la función de ventanas de SQL Server para obtener los resultados deseados de la siguiente manera:

CREATE TABLE [dbo].[EventStates](
    [Item] [varchar](1) NOT NULL,
    [EventDate] [varchar](19) NOT NULL,
    [Event] [varchar](3) NOT NULL
) ON [PRIMARY]

GO

INSERT [dbo].[EventStates] ([Item], [EventDate], [Event]) VALUES (N'A', N'2011-10-03 00:01:00', N'On')
INSERT [dbo].[EventStates] ([Item], [EventDate], [Event]) VALUES (N'A', N'2011-10-03 00:01:15', N'On')
INSERT [dbo].[EventStates] ([Item], [EventDate], [Event]) VALUES (N'B', N'2011-10-03 00:01:00', N'On')
INSERT [dbo].[EventStates] ([Item], [EventDate], [Event]) VALUES (N'A', N'2011-10-03 00:02:00', N'Off')
INSERT [dbo].[EventStates] ([Item], [EventDate], [Event]) VALUES (N'A', N'2011-10-03 00:02:01', N'Off')
INSERT [dbo].[EventStates] ([Item], [EventDate], [Event]) VALUES (N'C', N'2011-10-03 00:01:00', N'On')
INSERT [dbo].[EventStates] ([Item], [EventDate], [Event]) VALUES (N'B', N'2011-10-03 00:02:00', N'Off')
INSERT [dbo].[EventStates] ([Item], [EventDate], [Event]) VALUES (N'A', N'2011-10-03 00:02:02', N'On')
INSERT [dbo].[EventStates] ([Item], [EventDate], [Event]) VALUES (N'C', N'2011-10-03 00:02:05', N'On')
INSERT [dbo].[EventStates] ([Item], [EventDate], [Event]) VALUES (N'A', N'2011-10-03 00:02:07', N'Off')

GO

;WITH StateChange
AS
(
    SELECT   E.[Item]
            ,E.[EventDate]
            ,E.[Event]
            ,(  -- First determine if a state change to on has occurred.
                CASE
                    WHEN E.[Event] = 'On' AND LAG(E.[Event], 1, NULL) OVER (PARTITION BY E.[Item] ORDER BY E.[EventDate] ASC) IS NULL THEN 1
                    WHEN E.[Event] = 'On' AND E.[Event] <> LAG(E.[Event], 1, NULL) OVER (PARTITION BY E.[Item] ORDER BY E.[EventDate] ASC) THEN 1
                    ELSE 0
                END
             ) [StateChanged]
    FROM    EventStates E
), StateChangeGrouping
AS
(
    SELECT   [Item]
            ,[EventDate]
            ,[Event]
            ,[StateChanged]
            ,SUM([StateChanged]) OVER (PARTITION BY [Item] ORDER BY [EventDate] ASC) AS [GroupID]
    FROM    StateChange 
), StateChangeRanked
AS
(
    SELECT   [Item]
            ,[EventDate]
            ,[Event]
            ,[StateChanged]
            ,[GroupID]
            ,ROW_NUMBER() OVER (PARTITION BY [Item], GroupID, [Event] ORDER BY [EventDate]) AS TransitionRank
    FROM    StateChangeGrouping
)
SELECT      [Item]
            ,MIN([EventDate]) AS [Start]
            ,MAX([EventDate]) AS [End]
            ,[GroupID]
FROM        StateChangeRanked
WHERE       GroupID > 0 AND TransitionRank = 1
GROUP BY    [Item], GroupID
ORDER BY    [Item], [Start]

A continuación se muestra una implementación para SQL Server 2008.

;WITH EventID
AS
(
    SELECT   ROW_NUMBER() OVER(PARTITION BY [Item] ORDER BY [EventDate]) RowNr
            ,[Item]
            ,[EventDate]
            ,[Event] 
    FROM    [dbo].[EventStates]
), StateChange
AS
(
    SELECT  C.RowNr
            ,P.RowNr AS P_RowNr
            ,C.[Item]
            ,C.[EventDate]
            ,C.[Event] 
            ,P.[Event] AS P_Event
            ,(  -- First determine if a state change to on has occurred.
                CASE
                    WHEN C.[Event] = 'On' AND P.[Event] IS NULL THEN 1
                    WHEN C.[Event] = 'On' AND C.[Event] <> P.[Event] THEN 1
                    ELSE 0
                END
                ) [StateChanged]
    FROM    EventID C
    LEFT OUTER JOIN EventID P ON C.Item = P.Item AND C.RowNr = P.RowNr + 1
), StateChangeGrouping
AS
(
    SELECT   ST1.[Item]
            ,ST1.[EventDate]
            ,ST1.[Event]
            ,ST1.[StateChanged]
            ,(
                SELECT SUM([StateChanged])
                FROM StateChange ST2
                WHERE ST2.Item = ST1.Item AND ST2.RowNr <= ST1.RowNr
             ) AS GroupID
    FROM    StateChange ST1
), StateChangeRanked
AS
(
    SELECT   [Item]
            ,[EventDate]
            ,[Event]
            ,[StateChanged]
            ,[GroupID]
            ,ROW_NUMBER() OVER (PARTITION BY [Item], GroupID, [Event] ORDER BY [EventDate]) AS TransitionRank
    FROM    StateChangeGrouping
)
SELECT      [Item]
            ,MIN([EventDate]) AS [Start]
            ,MAX([EventDate]) AS [End]
            ,[GroupID]
FROM        StateChangeRanked
WHERE       GroupID > 0 AND TransitionRank = 1
GROUP BY    [Item], GroupID
ORDER BY    [Item], [Start]