Update 无法找到 TableMapping['a'] 或 DataTable“a”


private string search;
        string sql;           //数据库语句  根据不同选择   赋值不同
        public DataSet ds = new DataSet();
        SqlDataAdapter sda1 = null;
        public SqlConnection conn = new              SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings[1].ConnectionString.ToString());
    
private void BT_edit_Click(object sender, EventArgs e)
        {
            
            sda1.Update(ds, "a");
            conn.Close();
            Databind();
        }public void Databind()
        {
            if (RBT_name.Checked == true)                  //如果选中  按名称查找
            {
                search = TB_name.Text.Trim();
                sql = "select  [hw_name]  '名称',[hw_bianma] '产品编码' ,[hw_shuliang] '数量',[hw_weizhi] '位置'from [kucun] where [hw_name]='" + search + "'";
            }            if (RBT_bianma.Checked == true)
            {
                search = TB_bianma.Text.Trim();
                sql = "select  [hw_name]  '名称',[hw_bianma] '产品编码' ,[hw_shuliang] '数量',[hw_weizhi] '位置'from [kucun] where [hw_bianma]='" + search + "'";
            }
            if (RBT_weizhi.Checked == true)
            {
                search = CBOX_huojia.Text.Trim() + CBOX_ceng.Text.Trim();
                sql = "select   [hw_name]  '名称',[hw_bianma] '产品编码' ,[hw_shuliang] '数量', [hw_weizhi] '位置'from [kucun] where [hw_weizhi]='" + search + "'";
            }
            
            SqlCommand scmd1 = new SqlCommand(sql, conn);
            sda1 = new SqlDataAdapter(scmd1);
            sda1.SelectCommand = scmd1;
            SqlCommandBuilder scb = new SqlCommandBuilder(sda1);            
            sda1.UpdateCommand = scb.GetUpdateCommand();
            
            try
            {
                conn.Open();
                ds.Clear();
                sda1.Fill(ds, "a");
                this.DATA_chakan.DataSource = ds.Tables[0];
                ds.Tables.Clear();  // 将ds中的数据清空    要不下一个项目的查询   数据会重叠
                            }
            catch (Exception ex)
            {
                MessageBox.Show("数据库异常");
            }
            finally
            {
                conn.Close();
                scmd1.Dispose();
                sda1.Dispose();            }
        }