Finding the Difference of Two Results - sql

Question

I have two results with two different dates (a recent one and the previous one) the numbers below are the result 250 being the most recent and 300 being the previous result:
250
300
The code I use is here:
SELECT TOP 2
MY FIELD as bmi
FROM
MY TABLE
ORDER BY
THE DATE FIELD DESC
Within this same code I want to be able to find the difference between those two numbers and for that to appear not the two numbers?
I have tried a few things of skipping N rows etc but now I don't know what I can do?

Answer 1

I think you want something like this:
declare #firstBmiRes int
declare #secondBmiRes int
SET #firstBmiRes = 250 /* insert your query */
SET #secondBmiRes = 300 /* insert your query */
(SELECT SUM(#secondBmiRes - #firstBmiRes))
If you want to continue to use the calculated result. You can obviously store the value into another variable like this:
declare #bmi int
SET #bmi = (SELECT SUM(#secondBmiRes - #firstBmiRes))
SELECT #bmi
2nd Approach:
Since we don't have very much information to work with. you could try something like this... But i'm assuming a lot of your datastructure here.
declare #BmiScore int
declare #firstBmiRes int
declare #secondBmiRes int
SET #firstBmiRes = (SELECT TOP 1 MY_FIELD
FROM MY_TABLE
ORDER BY DATE_FIELD DESC)
SET #secondBmiRes = (SELECT MY_FIELD
FROM MY_TABLE
ORDER BY DATE_FIELD DESC
OFFSET 1 ROW
FETCH NEXT 1 ROW ONLY)
SET #bmiScore = (SELECT SUM(#secondBmiRes - #firstBmiRes))
SELECT #bmiScore

Answer 2

SELECT
MYFIELD - LAG (MYFIELD,1) OVER (ORDER BY MYDATE) AS BMI
FROM
MYTABLE;
ORDER BY MYDATE DESC

Answer 3

Using a LEAD function if you want your code to be a part of new code for some reason:
select TOP 1 (bmi - lead(bmi) over (order by date_field)) as result
from( SELECT TOP 2 my_field as bmi
, date_field
FROM my_table
ORDER BY date_field DESC) A
Here is a DEMO
Or by LAG :
select TOP 1 (lag(my_field) over (order by date_field) - my_field ) as result
FROM my_table
ORDER BY date_field DESC;

Answer 4

You can use LEAD/ LAG if your version of SQL Server supports these functions. If you are on an older version then you can use a windowed function to apply an order to the rows.
Here's your data going into a temporary table variable:
DECLARE #MY_TABLE TABLE (THE_DATE_FIELD DATE, MY_FIELD INT);
INSERT INTO #MY_TABLE SELECT '20200114', 300 UNION ALL SELECT '20200113', 250;
...and here's a query to perform the calculation you needed:
WITH x AS (
SELECT TOP 2
THE_DATE_FIELD,
MY_FIELD AS bmi,
ROW_NUMBER() OVER (ORDER BY THE_DATE_FIELD DESC) AS order_id
FROM
#MY_TABLE)
SELECT
MAX(CASE WHEN order_id = 1 THEN bmi END) - MAX(CASE WHEN order_id = 2 THEN bmi END) AS difference_bmi
FROM
x;
If I peek at the data from the CTE then I see this (and this is why I included the date field, which is redundant, and could otherwise be removed):
THE_DATE_FIELD bmi order_id
2020-01-14 300 1
2020-01-13 250 2
Now it's simply a case of picking the two values, as one has an order_id = 1 and one has an order_id = 2.