Calculate delta(difference of current and previous row) in sql

Question

I have a table like : trans is the name of the table for example

Id | Trans_Date          | Account_Id | Amount | Delta
------------------------------------         


        

Answer 1

Here's your original query modified accordingly:

select
  tt1.id,
  tt1.amount,
  tt1.AccountId,
  (tt1.amount-ifnull(tt2.amount, 0)) as delta
from trans tt1
  left outer JOIN trans tt2 on tt1.accountid = tt2.accountid
    and month(tt1.date1)-month(tt2.date1)=1;

The month comparison is moved from where to on, which makes a difference for left join, and tt2.amount is replaced with ifnull(tt2.amount, 0).

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The UPDATE version of the script:

update tt1
set delta = (tt1.amount-ifnull(tt2.amount, 0))
from trans tt1
  left outer JOIN trans tt2 on tt1.accountid = tt2.accountid
    and month(tt1.date1)-month(tt2.date1)=1;

---

The correct MySQL syntax for the above update should actually be:

update trans tt1 
             left outer JOIN trans tt2 
             on tt1.accountid = tt2.accountid 
             and month(tt1.date1)-month(tt2.date1)=1 
set tt1.delta = (tt1.amount-ifnull(tt2.amount, 0));

(Thanks @pinkb.)

Answer 2

You can use an inner query, but it's not necessarily the most efficient query.

UPDATE trans
SET Delta = Amount - 
(SELECT Amount FROM trans t1
WHERE t1.Trans_Date < trans.Trans_Date
ORDER BY t1.Trans_Date DESC LIMIT 1)

Answer 3

Can you "union all" your query with a query that simply selects the first item for each account with the initial balance set as the delta, and the ID of that record as the id for the delta record? The result would be ordered by ID. Dirty but is it applicable?