Question

I have one table called Orders

PurchaseID VARCHAR
purchaseDate DATETIME
purchasePrice FLOAT

I want to find the difference between the purchase price on one day versus another day – The puyrchaseID will be the same.(This is just an example table of course)

SELECT a.purchasePrice AS purchasePriceDay1 ,  b.purchasePrice AS  purchasePriceDay1
from Orders a , Orders b
where a.PurchaseID = b.PurchaseID

Would that actually work

Answer 1

SELECT 
    a.PurchaseID, 
    ABS(a.PurchaseID - b.PurchaseID) AS diff
FROM 
    PurchaseID a INNER JOIN PurchaseID b ON a.PurchaseID=b.PurchaseID
WHERE a.PurchaseID=? 
    AND a.purchaseDate=? 
    AND b.purchaseDate=?

Fill in the placeholders with the ID and dates to compare.

Consult your DBMS documentation to see if you have the math absolute function.