SQL query: Exclude partial duplicates from query result

Question

I have data from multiple sensors. Each record has own id, sensor id, timestamp and measured value. Like this:

|--id--|--id_sensor--|-------timestamp-------|--value--|
    1          1       '2017-08-23 10:00:00'     30
    2          1       '2017-08-23 10:02:00'     30
    3          1       '2017-08-23 10:04:00'     31
    4          1       '2017-08-23 10:06:00'     31
    5          1       '2017-08-23 10:08:00'     32
    6          2       '2017-08-23 10:00:00'     24
    7          2       '2017-08-23 10:01:00'     24
    8          2       '2017-08-23 10:02:00'     24
    9          2       '2017-08-23 10:03:00'     24
    10         2       '2017-08-23 10:04:00'     24
    11         2       '2017-08-23 10:05:00'     24
    12         2       '2017-08-23 10:06:00'     25

I would like to exclude record if the value did not change so the result should look like this:

|--id--|--id_sensor--|-------timestamp-------|--value--|
    1          1       '2017-08-23 10:00:00'     30
    3          1       '2017-08-23 10:04:00'     31
    5          1       '2017-08-23 10:08:00'     32
    6          2       '2017-08-23 10:00:00'     24
    12         2       '2017-08-23 10:06:00'     25

I have to be compatible with sql server 2000 :-( I would like to avoid using cursors if posible. Can anybody help?

Answer 1

I think the following may be closer to what you really want:

select t.*
from (select t.*,
             (select top 1 t2.value
              from t t2
              where t2.sensor = t.sensor and
                    t2.timestamp < t.timestamp
              order by t2.timestamp desc
             ) as prev_value
      from t
     ) t
where prev_value is null or prev_value <> value;

You really should upgrade to a supported version of SQL Server. Free versions are even available.