Update row using value from row before - tsql

Question

i'm looking for update row using value from row before.
I have something like this:
Group by
Value1
Value2
Value2 - expected result
1
0
20
20
1
3
x
23
1
5
x
28
1
2
x
30
2
0
30
30
2
5
x
35
2
2
x
37
Value2 = Value2 from row before + value 1 but column "Group by" is importand. If Value2 before is in another group then: Value2 = Value2
Can sameone explain mi how do this update statement? I tried using CTE with LAG function but i always fall in infinite loop.
Code for create table:
create table test
(
[GroupBy] int
, [Date] date
, [Value1] int
, [Value2] int
)
Inserting data:
INSERT INTO test ([GroupBy], [Date] [Value1], [Value2])
VALUES
(1, '2022-01-01', 0, 20),
(1, '2022-01-02', 3, NULL),
(1, '2022-01-03', 5, NULL),
(1, '2022-01-04', 2, NULL),
(2, '2022-01-01', 0, 30),
(2, '2022-01-02', 5, NULL),
(2, '2022-01-03', 2, NULL)
Primary key by: [GroupBy], [Date]

Answer 1

Check using LAG function
-- Using LAG function
-- https://learn.microsoft.com/sql/t-sql/functions/lag-transact-sql?view=sql-server-ver15&WT.mc_id=DP-MVP-5001699
;With MyCTE as (
SELECT
t.[date], t.GroupBy,t.Value1,Value2 = ISNULL(t.Value2,0),
PreValue2 = ISNULL(LAG(t.Value2,1,0) OVER (PARTITION BY t.GroupBy ORDER BY [date]),0)
FROM test t
)
SELECT [date], GroupBy, Value1, Value2, PreValue2, [Value1+PreValue2] = Value1+PreValue2
FROM MyCTE
Following the comment, maybe the original request was not well describe and what you need is not "Value2 from row before + value 1" but "SUM of all Value2 from all rows before + value 1"
In this case, check this solution
-- Value1 + total of all previous Value2
;With MyCTE as (
SELECT
t.[date], t.GroupBy,t.Value1,Value2 = ISNULL(t.Value2,0)
,TotalPreValue2 = SUM(ISNULL(t.Value2,0))
OVER (PARTITION BY t.GroupBy ORDER BY [date] ROWS BETWEEN UNBOUNDED PRECEDING and CURRENT ROW)
FROM test t
)
SELECT [date], GroupBy, Value1, Value2, TotalPreValue2, [Value1+TotalPreValue2] = Value1+TotalPreValue2
FROM MyCTE
GO
And if you need something else like "sum of all Value2 from ALL previous rows before + sum of all value1 from, previous rows" then check this
-- total of all previous Value1 + total of all previous Value2
;With MyCTE as (
SELECT
t.[date], t.GroupBy,t.Value1,Value2 = ISNULL(t.Value2,0)
,TotalPreValue2 = SUM(ISNULL(t.Value2,0))
OVER (PARTITION BY t.GroupBy ORDER BY [date] ROWS BETWEEN UNBOUNDED PRECEDING and CURRENT ROW)
,TotalPreValue1 = SUM(ISNULL(t.Value1,0))
OVER (PARTITION BY t.GroupBy ORDER BY [date] ROWS BETWEEN UNBOUNDED PRECEDING and CURRENT ROW)
FROM test t
)
SELECT [date], GroupBy, Value1, Value2, TotalPreValue2, TotalPreValue1, [TotalPreValue1+TotalPreValue2] = TotalPreValue1+TotalPreValue2
FROM MyCTE