Recursive Lag Column Calculation in SQL - sql

Question

I am trying to write a procedure that inserts calculated table data into another table.
The problem I have is that I need each row's calculated column to be influenced by the result of the previous row's calculated column. I tried to lag the calculation itself but this does not work!
Such as:
(Max is a function I created that returns the highest of two values)
Id Product Model Column1 Column2
1 A 1 5 =MAX(Column1*2, Lag(Column2))
2 A 2 2 =MAX(Column1*2, Lag(Column2))
3 B 1 3 =MAX(Column1*2, Lag(Column2))
If I try the above in SQL:
SELECT
Column1,
MyMAX(Column1,LAG(Column2, 1, 0) OVER (PARTITION BY Product ORDER BY Model ASC) As Column2
FROM Source
...it says column2 is unknown.
Output I get if I LAG the Column2 calculation:
Select Column1, MyMAX(Column1,LAG(Column1*2, 1, 0) OVER (PARTITION BY Product ORDER BY Model ASC) As Column2
Id Column1 Column2
1 5 10
2 2 10
3 3 6
Why 6 on row 3? Because 3*2 > 2*2.
Output that I want:
Id Column1 Column2
1 5 10
2 2 10
3 3 10
Why 10 on row 3? Because previous result of 10 > 3*2
The problem is I can't lag the result of Column2 - I can only lag other columns or calculations of them!
Is there a technique of achieving this with LAG or must I use Recursive CTE? I read that LAG succeeds CTE so I assumed it would be possible. If not, what would this 'CTE' look like?
Edit: Or alternatively - what else could I do to resolve this calculation?

Answer 1

Edit
In hindsight, this problem is a running partitioned maximum over Column1 * 2. It can be done as simply as
SELECT Id, Column1, Model, Product,
MAX(Column1 * 2) OVER (Partition BY Model, Product Order BY ID ASC) AS Column2
FROM Table1;
Fiddle
Original Answer
Here's a way to do this with a recursive CTE, without LAG at all, by joining on incrementing row numbers. I haven't assumed that your Id is contiguous, hence have added an additional ROW_NUMBER(). You haven't mentioned any partitioning, so haven't applied same. The query simply starts at the first row, and then projects the greater of the current Column1 * 2, or the preceding Column2
WITH IncrementingRowNums AS
(
SELECT Id, Column1, Column1 * 2 AS Column2,
ROW_NUMBER() OVER (Order BY ID ASC) AS RowNum
FROM Table1
),
lagged AS
(
SELECT Id, Column1, Column2, RowNum
FROM IncrementingRowNums
WHERE RowNum = 1
UNION ALL
SELECT i.Id, i.Column1,
CASE WHEN (i.Column2 > l.Column2)
THEN i.Column2
ELSE l.Column2
END,
i.RowNum
FROM IncrementingRowNums i
INNER JOIN lagged l
ON i.RowNum = l.RowNum + 1
)
SELECT Id, Column1, Column2
FROM lagged;
SqlFiddle here
Edit, Re Partitions
Partitioning is much the same, by just dragging the Model + Product columns through, then partitioning by these in the row numbering (i.e. starting back at 1 each time the Product or Model resets), including these in the CTE JOIN condition and also in the final ordering.
WITH IncrementingRowNums AS
(
SELECT Id, Column1, Column1 * 2 AS Column2, Model, Product,
ROW_NUMBER() OVER (Partition BY Model, Product Order BY ID ASC) AS RowNum
FROM Table1
),
lagged AS
(
SELECT Id, Column1, Column2, Model, Product, RowNum
FROM IncrementingRowNums
WHERE RowNum = 1
UNION ALL
SELECT i.Id, i.Column1,
CASE WHEN (i.Column2 > l.Column2)
THEN i.Column2
ELSE l.Column2
END,
i.Model, i.Product,
i.RowNum
FROM IncrementingRowNums i
INNER JOIN lagged l
ON i.RowNum = l.RowNum + 1
AND i.Model = l.Model AND i.Product = l.Product
)
SELECT Id, Column1, Column2, Model, Product
FROM lagged
ORDER BY Model, Product, Id;
Updated Fiddle