Flag Rows Occurring 2 Hours After Previous Flag

Question

I have some messy RFID data due to over sensitive antenna's. There is a physical process that tracks an RFID tag moving through different stations in a cycle. An item with an RFID tag can move through the cycle more than one time a day, but it is highly unlikely that it could start the cycle within a two hour window of that first read.

I am trying to either create a flag column to determine when the new cycle begins for an item or return a count of how many times an item has gone through a cycle.

Here is some sample data:

CREATE TABLE [dbo].[samplerfiddata](
    [Item] [nvarchar](50) NOT NULL,
    [Station_Type] [nvarchar](50) NOT NULL,
    [Station_Name] [nvarchar](50) NOT NULL,
    [Timestamp] [datetime2](7) NOT NULL,
    [Trying_to_Create_this_Flag_Column] [nvarchar](50) NOT NULL
);

INSERT [dbo].[samplerfiddata] ([Item], [Station_Type], [Station_Name], [Timestamp], [Trying_to_Create_this_Flag_Column]) VALUES (N'A', N'Decontamination', N'Decontamination', CAST(N'2020-10-10T06:30:00.0000000' AS DateTime2), N'1')
INSERT [dbo].[samplerfiddata] ([Item], [Station_Type], [Station_Name], [Timestamp], [Trying_to_Create_this_Flag_Column]) VALUES (N'A', N'Decontamination', N'Decontamination', CAST(N'2020-10-11T14:30:00.0000000' AS DateTime2), N'1')
INSERT [dbo].[samplerfiddata] ([Item], [Station_Type], [Station_Name], [Timestamp], [Trying_to_Create_this_Flag_Column]) VALUES (N'A', N'Washer', N'Washer', CAST(N'2020-10-11T14:45:00.0000000' AS DateTime2), N'0')
INSERT [dbo].[samplerfiddata] ([Item], [Station_Type], [Station_Name], [Timestamp], [Trying_to_Create_this_Flag_Column]) VALUES (N'A', N'Decontamination', N'Decontamination', CAST(N'2020-10-11T15:15:00.0000000' AS DateTime2), N'0')
INSERT [dbo].[samplerfiddata] ([Item], [Station_Type], [Station_Name], [Timestamp], [Trying_to_Create_this_Flag_Column]) VALUES (N'A', N'Other', N'Decontamination', CAST(N'2020-10-11T23:30:00.0000000' AS DateTime2), N'1')
INSERT [dbo].[samplerfiddata] ([Item], [Station_Type], [Station_Name], [Timestamp], [Trying_to_Create_this_Flag_Column]) VALUES (N'A', N'Washer', N'Washer', CAST(N'2020-10-12T00:15:00.0000000' AS DateTime2), N'0')
INSERT [dbo].[samplerfiddata] ([Item], [Station_Type], [Station_Name], [Timestamp], [Trying_to_Create_this_Flag_Column]) VALUES (N'A', N'Other', N'Decontamination', CAST(N'2020-10-12T00:45:00.0000000' AS DateTime2), N'0')
INSERT [dbo].[samplerfiddata] ([Item], [Station_Type], [Station_Name], [Timestamp], [Trying_to_Create_this_Flag_Column]) VALUES (N'A', N'Other', N'Decontamination', CAST(N'2020-10-13T16:00:00.0000000' AS DateTime2), N'1')
INSERT [dbo].[samplerfiddata] ([Item], [Station_Type], [Station_Name], [Timestamp], [Trying_to_Create_this_Flag_Column]) VALUES (N'A', N'Other', N'Decontamination', CAST(N'2020-10-13T16:30:00.0000000' AS DateTime2), N'0')
INSERT [dbo].[samplerfiddata] ([Item], [Station_Type], [Station_Name], [Timestamp], [Trying_to_Create_this_Flag_Column]) VALUES (N'A', N'Decontamination', N'Decontamination', CAST(N'2020-10-14T13:30:00.0000000' AS DateTime2), N'1')
INSERT [dbo].[samplerfiddata] ([Item], [Station_Type], [Station_Name], [Timestamp], [Trying_to_Create_this_Flag_Column]) VALUES (N'B', N'Other', N'Decontamination', CAST(N'2020-10-12T08:30:00.0000000' AS DateTime2), N'1')
INSERT [dbo].[samplerfiddata] ([Item], [Station_Type], [Station_Name], [Timestamp], [Trying_to_Create_this_Flag_Column]) VALUES (N'B', N'Decontamination', N'Decontamination', CAST(N'2020-10-12T14:30:00.0000000' AS DateTime2), N'1')
INSERT [dbo].[samplerfiddata] ([Item], [Station_Type], [Station_Name], [Timestamp], [Trying_to_Create_this_Flag_Column]) VALUES (N'B', N'Washer', N'Washer', CAST(N'2020-10-12T14:45:00.0000000' AS DateTime2), N'0')
INSERT [dbo].[samplerfiddata] ([Item], [Station_Type], [Station_Name], [Timestamp], [Trying_to_Create_this_Flag_Column]) VALUES (N'B', N'Decontamination', N'Decontamination', CAST(N'2020-10-12T15:15:00.0000000' AS DateTime2), N'0')
INSERT [dbo].[samplerfiddata] ([Item], [Station_Type], [Station_Name], [Timestamp], [Trying_to_Create_this_Flag_Column]) VALUES (N'B', N'Decontamination', N'Decontamination', CAST(N'2020-10-12T18:00:00.0000000' AS DateTime2), N'1')
INSERT [dbo].[samplerfiddata] ([Item], [Station_Type], [Station_Name], [Timestamp], [Trying_to_Create_this_Flag_Column]) VALUES (N'B', N'Washer', N'Washer', CAST(N'2020-10-13T18:15:00.0000000' AS DateTime2), N'0')
INSERT [dbo].[samplerfiddata] ([Item], [Station_Type], [Station_Name], [Timestamp], [Trying_to_Create_this_Flag_Column]) VALUES (N'B', N'Decontamination', N'Decontamination', CAST(N'2020-10-13T19:00:00.0000000' AS DateTime2), N'1')

In this data we have two different items moving through the cycle with all scenarios captured. The business logic for a new cycle is defined as when an item is read by an RFID Antenna (Station) where the Station Type = Decontamination or the Station Name = Decontamination and it has been more than two hours since the previous first read of the cycle.

Ultimately, I am trying to return a result set that looks like this:

How could this be completed?

Answer 1

In this case you can take advantage of LAG() function to get previous Timestamp value.

Accesses data from a previous row in the same result set without the use of a self-join starting with SQL Server 2012 (11.x). LAG provides access to a row at a given physical offset that comes before the current row. Use this analytic function in a SELECT statement to compare values in the current row with values in a previous row.

Then simply use a conditional sum, if DATEDIFF with current Timestamp >= 2 add 1 otherwise add 0 to the final result grouped by Item.

;WITH data AS
(
SELECT
  Item,
      CASE WHEN 
                DATEDIFF (hour, 
                          COALESCE(LAG(Timestamp) 
                                       OVER (PARTITION BY Item 
                                             ORDER BY Timestamp), 
                                   DATEADD(hour, -2, Timestamp)),
                          Timestamp) >= 2 THEN 1 ELSE 0 END AS lastTM
FROM
  samplerfiddata
)
SELECT
  Item,
  SUM(lastTM) as Cycles
FROM
  data
GROUP BY
  Item;

Item | Cycles
:--- | -----:
A    |      5
B    |      4

db<>fiddle here