Question

Let’s say i have a table with the following data:

day1 item1 30
day1 item2 25
day1 item3 27
day2 item1 30
day2 item2 30
day2 item3 20
…
dayN item1 20

How would I structure a query that can give me the rate of change for each item on a daily basis.

day1 item1 rateOfChange:0
day1 item2 rateOfChange:-1
day1 item3 rateOfChange:5
day2 item1 rateOfChange:0
day2 item2 rateOfChange:2

My inital thought (without knowing sql too well), would be to loop through each day, and have a subquery that checks for the day before for each item and does a difference?

Thanks

Answer 1

If you table was called datatable with a day,item and value integer columns, this might do it

SELECT t1.day, t1.item, (t2.value - t1.value) as rate
FROM datatable t1
INNER JOIN datatable t2
ON t1.item = t2.item 
AND t1.day + 1 = t2.day