How to calculate the slope in SQL

Question

I have some data in a sql database and I\'d like to calculate the slope. The data has this layout:

Date        |  Keyword  |  Score    
2012-01-10  |  ipad          


        

Answer 1

If you're defining slope as just the slope from the earliest point to the latest point, and if score only increases with date, then you can get the output above with this:

SELECT *
  FROM scores
  JOIN
    (SELECT foo.keyword,
            (MAX(score)-MIN(score)) / DATEDIFF(MAX(date),MIN(date)) AS score
     FROM scores
     GROUP BY keyword) a
  USING(keyword);

However if you want linear regression, or if scores can decrease as well as increase with time, you'll need something more complex.

Answer 2

Use this

SUM(CONVERT(float, datediff(dd, '1/1/1900', date_field)))

instead of

SUM(CAST(date_field AS float))

Answer 3

The cleanest one I could make:

SELECT
    Scores.Date, Scores.Keyword, Scores.Score,
    (N * Sum_XY - Sum_X * Sum_Y)/(N * Sum_X2 - Sum_X * Sum_X) AS Slope
FROM Scores
INNER JOIN (
    SELECT
        Keyword,
        COUNT(*) AS N,
        SUM(CAST(Date as float)) AS Sum_X,
        SUM(CAST(Date as float) * CAST(Date as float)) AS Sum_X2,
        SUM(Score) AS Sum_Y,
        SUM(Score*Score) AS Sum_Y2,
        SUM(CAST(Date as float) * Score) AS Sum_XY
    FROM Scores
    GROUP BY Keyword
) G ON G.Keyword = Scores.Keyword;

It uses Simple Linear Regression to calculate the slope.

Result:

Date         Keyword        Score         Slope
2012-01-22   water bottle   0,010885442   0,000334784345222076
2012-01-23   water bottle   0,011203949   0,000334784345222076
2012-01-24   water bottle   0,008460835   0,000334784345222076
2012-01-25   water bottle   0,010363991   0,000334784345222076
2012-01-26   water bottle   0,011800716   0,000334784345222076
2012-01-27   water bottle   0,012948411   0,000334784345222076
2012-01-28   water bottle   0,012732459   0,000334784345222076
2012-01-29   water bottle   0,011682568   0,000334784345222076

Every database system seems to have a different approach to converting dates to numbers:

• MySQL: TO_SECONDS(date) or TO_DAYS(date)
• Oracle: TO_NUMBER(TO_CHAR(date, 'J')) or date - TO_DATE('1','yyyy')
• MS SQL Server: CAST(date AS float) (or equivalent CONVERT)

Answer 4

Cast to decimal does not give correct results for me, it is not linear to the dates. Use TO_DAYS(date_field) instead, this becomes correct.