What putting a query something like this into the String and executing it ? (Check the syntax as this is done without checking).

Code:
--- Not quite sure if you need this line but put it in anyway.
SET NOCOUNT ON

DECLARE @SDInputDate Integer

SET @SDInputDate = SELECT SDInputDate FROM Stock_Details WHERE ObjectID = 4887

--- Switch it Off
SET NOCOUNT OFF

UPDATE Stock_Details SET SDInputDate = @SDInputDate WHERE ObjectID = 504

Chubby.