Updating fields from two tables of Sql CE in C#? -

i have 2 tables in stock.sdf database called:

product (pid, pname, unitprice) sales (pid, qty, totalprice) 

i want add totalprice sales table using update query. because want store data permanently, whenever new sales added.

 sales.totalprice = (sales.qty * product.unitprice sales.pid = product.pid); 

i have tried follows , got parsing error. possible solution

try             {                 stockdbconnection dbcon = new stockdbconnection();                 sqlceconnection conn = new sqlceconnection(dbcon.returnconnection("connstring"));                 conn.open();                 sqlcecommand command = new sqlcecommand("update set a.tprice=@qty * b.uprice sales  join product b on @pid=b.pid", conn);                 command.parameters.add("@qty", txtqty.text);                 command.parameters.add("@pid", cmbpid.text);                 command.executenonquery();                 messagebox.show("data upated successfully.");             }              catch (exception exp)             {                 messagebox.show(exp.message);             } 

try query

update set a.totalprice=a.qty * b.unitprice sales  join product b on a.pid=b.pid