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UPDATE Row using values from prior row

Hello,
I have this:

UPDATE MONEY SET Balance = Credit - Debit + "X"

Ok, that "X" is supposed to be the prior row. I need to calculate the Balance, which is the result of the previous balance + the result of credit + debit. The thing is by the nature of our business we need to alter some old rows and recalculate the balance. The update works that way? How to get the value for a column in the previous row?

thanks.
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starusa
Asked:
starusa
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2 Solutions
 
Kevin CrossChief Technology OfficerCommented:
Do you have an incrementing primary key (PK) or some other column that indicates a row is older than the one you are on?  If so, you could do something like this:


UPDATE MONEY 
SET Balance = Credit - Debit + 
IsNull((SELECT b.balance FROM MONEY b WHERE b.pkColumn IN (SELECT max(pkColumn) FROM MONEY c WHERE c.pkColumn < MONEY.pkColumn)), 0.00)

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BrandonGalderisiCommented:
I would POSSIBLY suggest not storing the balance then, if values are subject to change.  But that can be figured out

Or, you will have to run this frequently:

;with CurrentPast as
(select AccountNo, TransDate, Credit, Debit, cast(Null as money) as PreviousCredit, cast(Null as money) as PreviousDebit, row_number() over (partition by AccountNo order by TransDate) rn
from Money
union all
select mpAccountNo, mp.TransDate, mp.Credit, mp.Debit, row_number() over (partition by AccountNo order by TransDate) rn
from Money m
join CurrentPast cp
on m.accountNo = cp.AccountNo
and m.rn-1 = cp.rn)
select * from CurrentPast


^^ This will give you the each transaction as pairs. ^^


;with CurrentPast as
(select AccountNo, TransDate, Credit, Debit, cast(credit-debit as money) as CurrentChange, cast(0 as money) as PreviousBalance, row_number() over (partition by AccountNo order by TransDate) rn
from Money
union all
select mp.AccountNo, mp.TransDate, mp.Credit, mp.Debit, row_number() over (partition by AccountNo order by TransDate) rn
from Money m
join CurrentPast cp
on m.accountNo = cp.AccountNo
and m.rn-1 = cp.rn)



Now this is some sample code that shows you how to show a Running balance without ever changing it.  And it allows you to see a balance as of a point in time:



set nocount on
create table #tabMoney
(accountNo     int
,transDate     datetime
,Credit        money
,Debit         money
)
insert into #TabMoney values(1,'1/1/2007',0,20)
insert into #TabMoney values(1,'1/2/2007',20,0)
insert into #TabMoney values(1,'1/3/2007',0,15)
insert into #TabMoney values(1,'1/4/2007',10,0)
 
insert into #TabMoney values(2,'1/1/2007',0,100)
insert into #TabMoney values(2,'2/2/2007',0,50)
insert into #TabMoney values(2,'3/3/2007',160,0)
insert into #TabMoney values(2,'4/1/2007',0,10)
go
;with CurrentPast as
(
select t.* from 
(select AccountNo
     ,TransDate
     ,Credit
     ,Debit
     ,credit-debit as CurrentChange
     ,credit-debit as CurrentBalance
     ,row_number() over (partition by AccountNo order by TransDate) as rn
from #tabMoney) t
where t.rn=1
union all
select m.AccountNo, m.TransDate, m.Credit, m.Debit, m.CreditDebit, cp.CurrentChange+CreditDebit, m.rn
from 
     (select AccountNo
          ,TransDate
          ,Credit
          ,Debit
          ,credit-debit CreditDebit
          ,row_number() over (partition by AccountNo order by TransDate) rn
     from #tabMoney) m
  inner join CurrentPast cp
    on  m.accountNo = cp.AccountNo
    and m.rn-1 = cp.rn
)
select * from CurrentPast
Order by AccountNo,rn
go
drop table #tabMoney

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BrandonGalderisiCommented:
There are some assumptions based upon field names and structures.  But it can be easily adapted if you table structure is nothing like this.
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Kevin CrossChief Technology OfficerCommented:
I would agree with Brandon on not storing if you don't have to and if you want to use a common table expression and do have SQL 2005 you can modify the first query I gave you to do this:
WITH moneyCTE AS (
    SELECT *,
    row_number() over (ORDER BY transdate) AS rn
    FROM Money
)
UPDATE a
SET a.Balance = a.Credit - a.Debit + IsNull(b.balance, 0.00)
FROM moneyCTE a LEFT JOIN moneyCTE b
ON a.rn-1 = b.rn

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Kevin CrossChief Technology OfficerCommented:
As Brandon, is saying here is he same query as a select instead of an update.
WITH moneyCTE AS (
    SELECT *,
    row_number() over (ORDER BY transdate) AS rn
    FROM Money
)
SELECT *, -- you can list out columns here
(a.Credit - a.Debit + IsNull(b.balance, 0.00)) AS newBalance
FROM moneyCTE a LEFT JOIN moneyCTE b
ON a.rn-1 = b.rn

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BrandonGalderisiCommented:
Same basic principal except I contained all of logic within the CTE to simplify the select.  6 & 1/2 dozen :)
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Kevin CrossChief Technology OfficerCommented:
True.  Was just extending my query and at first glance your query looked much different with the union all statement, but I see now where you were going with that.

Regards,
Kev
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