AutoIncrement column based of FK

I have a table


 PK                                      FK                            
 OrderDetailID                OrderID           LineNo          
 8238                                7780                  1
 8239                                7780                  2
 8240                                7781                  1

 How do I create the column LineNo based of OrderID to increment starting with one?  So once the FK OrderID changes it starts back to 1 for the next orderid.  The lineno field must autogenerate and needs to be stored.
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Éric MoreauSenior .Net ConsultantCommented:
why do you need to store it if you can easily generate it?
Éric MoreauSenior .Net ConsultantCommented:
another issue you might have if you store that value is that if you remove rows from your table, you will get holes in your LineNo sequence
JDay2Author Commented:
I need to store it because it will be tied with order detail information and the line number makes it unique.

I don't want to use the PK field and the viewing of line no per order seems logical.

I will have it so you cannot remove rows and have an option to cancel as a status.
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Éric MoreauSenior .Net ConsultantCommented:
it would be easier if your client application handling the orders would provide that value.
Éric MoreauSenior .Net ConsultantCommented:
JDay2Author Commented:
can I make this a stored procedure to calculate?
Éric MoreauSenior .Net ConsultantCommented:
How do you add rows to your table?
JDay2Author Commented:
by sql business rule for that field in .net c#
Vitor MontalvãoMSSQL Senior EngineerCommented:
The only way I'm seeing to do what you want is with a trigger:
ON dbo.OrderDetail
	WITH CTE_RowNumber
	AS (
        select OrderDetailID, OrderID, ROW_NUMBER ( )  OVER (PARTITION BY OrderID order by OrderDetailID)  as RowNumber
        from OrderDetail
	SET t.Lineno = r.RowNumber
	FROM OrderDetail t
		INNER JOIN inserted i ON t.OrderDetailID = i.OrderDetailID
		INNER JOIN CTE_RowNumber r ON t.OrderDetailID = r.OrderDetailID

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And then run a dummy update to get all rows updated:
UPDATE OrderDetail 
SET OrderID = OrderID 

FROM OrderDetail 

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Éric MoreauSenior .Net ConsultantCommented:
If you are not calling a Stored Proc to do the insert, you cannot have a SP to set the value! That's why the article of sqlteam is suggesting a trigger.

If your .Net code is inserting the data directly, better have the .Net code to set the LineNo value as well.
Vitor MontalvãoMSSQL Senior EngineerCommented:
JDay2, a feedback will be appreciated.
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