Question

I have a SQL Server 2008 database with a history table for recording changes in a main table. I need to report the current (latest) value of the ‘rate’ column, and the most recent previous value that’s different from current.

So, given something like:

id   | rate   | uninteresting | updated_on   | version
-----+--------+---------------+--------------+----------
123  | 1.20   | foo           | 2010-10-18   | 1500
456  | 2.10   | bar           | 2010-10-12   | 2123
123  | 1.20   | baz           | 2010-10-10   | 1499
123  | 1.10   | baz           | 2010-10-08   | 1498
456  | 2.00   | bar           | 2010-10-11   | 2122
123  | 1.00   | baz           | 2010-08-01   | 1497
456  | 2.00   | quux          | 2010-10-05   | 2121
456  | 1.95   | quux          | 2010-09-07   | 2120

I want to produce:

id   | cur_rate | cur_ver | updated_on | prev_rate | prev_ver | prev_updated
-----+----------+---------+------------+-----------+----------+-------------
123  | 1.20     | 1500    | 2010-10-18 | 1.10      | 1498     | 2010-10-08
456  | 2.10     | 2123    | 2010-10-12 | 2.00      | 2122     | 2010-10-11

Note that I’m looking for the latest entry where the rate is different from the most recent entry.

I’ve tried various approaches, but either get way too many results, or none at all. Any suggestions?

Answer 1

There are a couple of ways to accomplish this. Here’s one way

Declare @table  as table( 
id int, 
rate decimal(10,5) , 
uninteresting  varchar(10) ,
updated_on  date, 
version int )
INSERT INTO @table
VALUES 
(123  , 1.20   , 'foo     ' , '2010-10-18'   , 1500),
(456, 2.1, ' bar           ', ' 2010-10-12   ', 2123),
(123, 1.2, ' baz           ', ' 2010-10-10   ', 1499),
(123, 1.1, ' baz           ', ' 2010-10-08   ', 1498),
(456, 2, ' bar           ', ' 2010-10-11   ', 2122),
(123, 1, ' baz           ', ' 2010-08-01   ', 1497),
(456, 2, ' quux          ', ' 2010-10-05   ', 2121),
(456, 1.95, ' quux          ', ' 2010-09-07   ', 2120)


;WITH rates 
     AS (SELECT Row_number() OVER ( PARTITION BY curr.id, curr.rate ORDER BY curr.updated_on DESC) AS rn,
            curr.id, 
            curr.rate        cur_rate, 
            curr.version     cur_ver, 
            curr.updated_on, 
            previous.rate       prev_rate, 
            previous.version    prev_ver, 
            previous.updated_on prev_updated 
          FROM 
                @table curr
                LEFT JOIN @table previous
                ON curr.id = previous.id 
                   AND curr.rate <> previous.rate
                   AND curr.updated_on > previous.updated_on 

   ) 
   SELECT 
           id, 
           cur_rate, 
           cur_ver, 
           updated_on, 
           prev_rate, 
           prev_ver, 
           prev_updated 
    FROM 
           rates 
    WHERE 
           rn = 1

produces this result

id          cur_rate cur_ver     updated_on prev_rate prev_ver    prev_updated
----------- -------- ----------- ---------- --------- ----------- ------------
123         1.00000  1497        2010-08-01 NULL      NULL        NULL
123         1.10000  1498        2010-10-08 1.00000   1497        2010-08-01
123         1.20000  1500        2010-10-18 1.10000   1498        2010-10-08
456         1.95000  2120        2010-09-07 NULL      NULL        NULL
456         2.00000  2122        2010-10-11 1.95000   2120        2010-09-07
456         2.10000  2123        2010-10-12 2.00000   2122        2010-10-11

IF you change the rn to drop the rate in the partition by e.g.
( PARTITION BY curr.id ORDER BY curr.updated_on DESC) AS rn,

you get

id          cur_rate cur_ver     updated_on prev_rate prev_ver    prev_updated
----------- -------- ----------- ---------- --------- ----------- ------------
123         1.20000  1500        2010-10-18 1.10000   1498        2010-10-08
456         2.10000  2123        2010-10-12 2.00000   2122        2010-10-11