Question

I have a problem with a query.

This is the data (order by Timestamp):

Data
    ID  Value   Timestamp
    1   0       2001-1-1
    2   0       2002-1-1
    3   1       2003-1-1
    4   1       2004-1-1
    5   0       2005-1-1
    6   2       2006-1-1
    7   2       2007-1-1
    8   2       2008-1-1

I need to extract distinct values and the first occurance of the date. The exception here is that I need to group them only if not interrupted with a new value in that timeframe.
So the data I need is:

ID  Value   Timestamp
1   0       2001-1-1
3   1       2003-1-1
5   0       2005-1-1
6   2       2006-1-1

I’ve made this work by a complicated query, but am sure there is an easier way to do it, just cant think of it. Could anyone help?

This is what I started with – probably could work with that. This is a query that should locate when a value is changed.

  > SELECT * FROM Data d1 join Data d2 ON d1.Timestamp < d2.Timestamp and
    > d1.Value <> d2.Value

It probably could be done with a good use of row_number clause but cant manage it.

Answer 1

Sample data:

declare @T table (ID int, Value int, Timestamp date)
insert into @T(ID,  Value,   Timestamp) values
(1,   0,       '20010101'),
(2,   0,       '20020101'),
(3,   1,       '20030101'),
(4,   1,       '20040101'),
(5,   0,       '20050101'),
(6,   2,       '20060101'),
(7,   2,       '20070101'),
(8,   2,       '20080101')

Query:

;With OrderedValues as (
    select *,ROW_NUMBER() OVER (ORDER By TimeStamp) as rn --TODO - specific columns better than *
    from @T
), Firsts as (
    select
        ov1.* --TODO - specific columns better than *
    from
        OrderedValues ov1
            left join
        OrderedValues ov2
            on
                ov1.Value = ov2.Value and
                ov1.rn = ov2.rn + 1
    where
        ov2.ID is null
)
select * --TODO - specific columns better than *
from Firsts

I didn’t rely on the ID values being sequential and without gaps. If that’s the situation, you can omit OrderedValues (using the table and ID in place of OrderedValues and rn). The second query simply finds rows where there isn’t an immediate preceding row with the same Value.

Result:

ID          Value       Timestamp  rn
----------- ----------- ---------- --------------------
1           0           2001-01-01 1
3           1           2003-01-01 3
5           0           2005-01-01 5
6           2           2006-01-01 6

You can order by rn if you need the results in this specific order.