Update Query - Sum of previous row x and current row y - sql

Question

I having a table LedgerData and need to update the Balance.
Table Structure
CREATE TABLE [dbo].[LedgerData]
(
[Id] INT NOT NULL,
[CustomerId] INT NOT NULL,
[Credit] INT,
[Debit] INT,
[Balance] INT
CONSTRAINT [PK_dbo_LedgerData]
PRIMARY KEY CLUSTERED ([Id] ASC)
WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF,
IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON,
ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY];
Sample Data
INSERT INTO [dbo].[LedgerData] VALUES (1, 1, 50, 0, 0);
INSERT INTO [dbo].[LedgerData] VALUES (2, 1, 0, 25, 0);
INSERT INTO [dbo].[LedgerData] VALUES (3, 2, 0, 75, 0);
INSERT INTO [dbo].[LedgerData] VALUES (4, 1, 0, 10, 0);
INSERT INTO [dbo].[LedgerData] VALUES (5, 2, 5, 0, 0);
INSERT INTO [dbo].[LedgerData] VALUES (6, 1, 10, 25, 0);
I tried to update the balance column customer wise ORDER BY [Id] ASC, but its not updating as expected. Also I explored the sql query to calculate sum and add sum from previous rows
Please assist me to calculate the balance column Balance = (Previous Row Balance + Credit - Debit)

Answer 1

Ideally this is something you should be doing as you INSERT the data, by getting the previous value (and locking the table so that other INSERT statements can't occur to avoid races) and then supplying a value for the Balance. You can, however, UPDATE all the rows with a cumulative SUM and an updatable CTE:
WITH CTE AS(
SELECT ID,
CustomerID,
Balance,
0 + SUM(Credit) OVER (PARTITION BY CustomerID ORDER BY ID) - SUM(Debit) OVER (PARTITION BY CustomerID ORDER BY ID) AS NewBalance
FROM dbo.LedgerData)
UPDATE CTE
SET Balance = NewBalance;
GO
SELECT *
FROM dbo.LedgerData;
Alternatively, don't store the aggregate value at all, and use a VIEW so that the value can always be calculated (accurately) with the same expression I have used in the CTE. For example:
CREATE VIEW dbo.LedgerDataCumulative
AS
SELECT Id,
CustomerId,
Credit,
Debit,
SUM(Credit) OVER (PARTITION BY CustomerID ORDER BY ID) - SUM(Debit) OVER (PARTITION BY CustomerID ORDER BY ID) AS Balance
FROM dbo.LedgerData;
GO

Answer 2

The update can be performed with a single window function.
with upd_cte as (
select *, sum([Credit]-[Debit]) over (partition by customerId order by id) sum_over
from #LedgerData)
update upd_cte
set Balance=sum_over;