calculating percent change over time - sql

Question

I've created the structure and sample data here. I'm not sure how to go about calculating the change over time.
My desired result set is:
a | % growth
abc | 4.16
def | 0.83
hig | -0.2
The % change being (last value - first value) / days:
a | % growth
abc | (30-5) / 6
def | (6-1) / 6
hig | (4-5) / 5
I'm trying:
SELECT a.*,
b.val,
c.val
FROM (SELECT a,
Min(dt) AS lowerDt,
Max(dt) AS upperDt
FROM tt
GROUP BY a) a
LEFT JOIN tt b
ON b.dt = a.lowerdt
AND b.a = a.a
LEFT JOIN tt c
ON c.dt = a.upperdt
AND b.a = a.a
If possible, I'd like to avoid a CTE.

Answer 1

You don't want min and max, you really want first and last.
One way I do that is to use ROW_NUMBER() to tell me the position from the begining or the end. Then use MAX(CASE WHEN pos=1 THEN x ELSE null END) to get the values I want.
SELECT
a,
MAX(CASE WHEN pos_from_first = 1 THEN dt ELSE NULL END) AS first_date,
MAX(CASE WHEN pos_from_final = 1 THEN dt ELSE NULL END) AS final_date,
MAX(CASE WHEN pos_from_first = 1 THEN val ELSE NULL END) AS first_value,
MAX(CASE WHEN pos_from_final = 1 THEN val ELSE NULL END) AS final_value,
100
*
CAST(MAX(CASE WHEN pos_from_final = 1 THEN val ELSE NULL END) AS DECIMAL(9,6))
/
CAST(MAX(CASE WHEN pos_from_first = 1 THEN val ELSE NULL END) AS DECIMAL(9,6))
-
100 AS perc_change
FROM
(
SELECT
ROW_NUMBER() OVER (PARTITION BY a ORDER BY dt ASC) AS pos_from_first,
ROW_NUMBER() OVER (PARTITION BY a ORDER BY dt DESC) AS pos_from_final,
*
FROM
tt
)
AS ordered
GROUP BY
a
http://sqlfiddle.com/#!6/ad95d/11