Creating on change function

Question

How to write a on change function selected dropdownlist values and Gridvalue one column


my select Query is   string strSQL = "Select a.facreceiptitemid ,a.itemid,a.batchno,to_char(a.mfgdate,'mm/yyyy') as mfgdate,to_char(a.expdate,'mm/yyyy') as expdate,a.MRP,a.SingleUnitPrice,a.marginPercentage,a.sellingprice,a.saletaxid," +

                        "a.TaxPercentage,a.SaleTaxPercentage,a.SaleTaxType,a.sellingpricesku from tbfacilityreceiptitems a " +

                        "inner join masitems it on (it.itemid = a.itemid)" +

                        "inner join masitemcategories cat on (cat.categoryid = it.categoryid)" +

                        "where cat.categoryid =" + ddlCategory;
Please help..How to write a onchange function for this query