sql - Query for sum of column values not giving me a value -

i have gridview in asp page user can change 1 of values , other read value. want when row updated query execute sums value of 1 column other , changes it. ex... value1 + value 2 = value2. wrote in code-behind:

    protected sub gridview1_rowupdated(sender object, e eventargs) handles gridview1.rowupdated     dim connectionstring string = "mystring"     using cn new sqlconnection(connectionstring)         dim cmd new sqlcommand()         cmd = new sqlcommand("select (purchased + actual) actual product", cn)           cn.open()         cmd.executenonquery()         cn.close()     end using 

yet value doesn't sum. part of me says need update command save value or read in w3 , other places doesn't point me that. guess since there's different products (of course) need productid = @parameter... right? still code should change rest of values , doesn't. pointers or flew on me? bunch!

looks want

update product set value2 = value1 + value2  productid = @parameter