Trying to find the last time that a value has changed

I have a table that has an ID, a value, and a date. There are many IDs, Values, and dates in this table.

Records are inserted into this table periodically. The ID will always stay the same but occasionally the value will change.

How can I write a query that will give me the ID plus the most recent time the value has changed? Note: the value will always increase.

From this sample data:

Create Table Taco
 (  Taco_ID int,
    Taco_value int,
    Taco_date datetime)

Insert INTO Taco 
Values (1, 1, '2012-07-01 00:00:01'),
        (1, 1, '2012-07-01 00:00:02'),
        (1, 1, '2012-07-01 00:00:03'),
        (1, 1, '2012-07-01 00:00:04'),
        (1, 2, '2012-07-01 00:00:05'),
        (1, 2, '2012-07-01 00:00:06'),
        (1, 2, '2012-07-01 00:00:07'),
        (1, 2, '2012-07-01 00:00:08')

The result should be:

Taco_ID      Taco_date
1            2012-07-01 00:00:05

(Because 00:05 was the last time Taco_Value changed.)

These two queries rely on the assumption that Taco_value always increases over time.

;WITH x AS
(
  SELECT Taco_ID, Taco_date,
    dr = ROW_NUMBER() OVER (PARTITION BY Taco_ID, Taco_Value ORDER BY Taco_date),
    qr = ROW_NUMBER() OVER (PARTITION BY Taco_ID ORDER BY Taco_date)
  FROM dbo.Taco
), y AS
(
  SELECT Taco_ID, Taco_date,
    rn = ROW_NUMBER() OVER (PARTITION BY Taco_ID, dr ORDER BY qr DESC)
  FROM x WHERE dr = 1
)
SELECT Taco_ID, Taco_date
FROM y 
WHERE rn = 1;

An alternative with fewer window function madness:

;WITH x AS
(
  SELECT Taco_ID, Taco_value, Taco_date = MIN(Taco_date)
  FROM dbo.Taco
  GROUP BY Taco_ID, Taco_value
), y AS
(
  SELECT Taco_ID, Taco_date, 
    rn = ROW_NUMBER() OVER (PARTITION BY Taco_ID ORDER BY Taco_date DESC)
  FROM x
)
SELECT Taco_ID, Taco_date FROM y WHERE rn = 1;

Examples at SQLfiddle

Update

For those keeping track, there was contention over what happens if Taco_value could ever repeat. If it could go from 1 to 2 and then back to 1 for any given Taco_ID, the queries will not work. Here is a solution for that case, even if it isn't quite the gaps & islands technique that someone like Itzik Ben-Gan may be able to dream up, and even if it isn't relevant for the OP's scenario - it may be relevant to a future reader. It's a little more complex, and I also added an additional variable - a Taco_ID that only ever has one Taco_value.

If you want to include the first row for any ID where value didn't change at all in the entire set:

;WITH x AS
(
  SELECT *, rn = ROW_NUMBER() OVER 
    (PARTITION BY Taco_ID ORDER BY Taco_date DESC)
  FROM dbo.Taco
), rest AS (SELECT * FROM x WHERE rn > 1)
SELECT  
  main.Taco_ID, 
  Taco_date = MIN(CASE 
    WHEN main.Taco_value = rest.Taco_value 
    THEN rest.Taco_date ELSE main.Taco_date 
  END)
FROM x AS main LEFT OUTER JOIN rest
ON main.Taco_ID = rest.Taco_ID AND rest.rn > 1
WHERE main.rn = 1
AND NOT EXISTS 
(
  SELECT 1 FROM rest AS rest2
   WHERE Taco_ID = rest.Taco_ID
   AND rn  rest.Taco_value
) 
GROUP BY main.Taco_ID;

If you want to exclude those rows, it's a bit more complex, but still minor changes:

;WITH x AS
(
  SELECT *, rn = ROW_NUMBER() OVER 
    (PARTITION BY Taco_ID ORDER BY Taco_date DESC)
  FROM dbo.Taco
), rest AS (SELECT * FROM x WHERE rn > 1)
SELECT 
  main.Taco_ID, 
  Taco_date = MIN(
  CASE 
    WHEN main.Taco_value = rest.Taco_value 
    THEN rest.Taco_date ELSE main.Taco_date 
  END)
FROM x AS main INNER JOIN rest -- ***** change this to INNER JOIN *****
ON main.Taco_ID = rest.Taco_ID AND rest.rn > 1
WHERE main.rn = 1
AND NOT EXISTS
(
  SELECT 1 FROM rest AS rest2
   WHERE Taco_ID = rest.Taco_ID
   AND rn  rest.Taco_value
)
AND EXISTS -- ***** add this EXISTS clause ***** 
(
  SELECT 1 FROM rest AS rest2
   WHERE Taco_ID = rest.Taco_ID
   AND Taco_value <> rest.Taco_value
)
GROUP BY main.Taco_ID;

Updated SQLfiddle examples

StackExchange Database Administrators Q#44784, answer score: 13