SQL server 2005 - grouping values, display only ch

问题:

I'm trying to group a set of values, but I only want to display the values if there was a change.

Some example data

value | date_loaded | 
  1   |  2012-03-07 |
  1   |  2012-03-06 |
  1   |  2012-03-05 |
  3   |  2012-03-04 |
  4   |  2012-03-03 |
  1   |  2012-03-02 |

So I would like to display the latest value, with it's earliest date, e.g.

value | date_loaded | 
  1   |  2012-03-05 |
  3   |  2012-03-04 |
  4   |  2012-03-03 |
  1   |  2012-03-02 |

What is the best way to go about this? Would it be possible to do an if statement? If value 1 is different to value 2, +1 to "change" ? Therefore I'd be able to group the values by "change1", "change2" etc.?

回答1:

I would use a common table expression to include a sequential row number based on date_loaded, and then use that in a self-join, as follows:

CREATE TABLE #temp (
    value INT,
    date_loaded DATETIME
)

INSERT INTO #temp VALUES (1,'2012-03-7')
INSERT INTO #temp VALUES (1,'2012-03-6')
INSERT INTO #temp VALUES (1,'2012-03-5')
INSERT INTO #temp VALUES (3,'2012-03-4')
INSERT INTO #temp VALUES (4,'2012-03-3')
INSERT INTO #temp VALUES (1,'2012-03-2')

;WITH cte AS (SELECT ROW_NUMBER() OVER (ORDER BY date_loaded) AS n, value, date_loaded FROM #temp)
SELECT t2.value, t2.date_loaded
FROM cte t2 LEFT JOIN cte t1 ON t2.n = t1.n + 1
WHERE t2.value <> ISNULL(t1.value, -1)

DROP TABLE #temp