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).

---

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.)