sql = "select distinct * from planeinformation";
            da = new SqlDataAdapter(sql,DBHelper.Connection);
            st = new DataSet();
            da.Fill(st,"start");

            DataRow row = st.Tables["start"].NewRow();
            row["id"] = -1;
            row["setout"] = "请选择";
            st.Tables["start"].Rows.InsertAt(row,0);
            
            this.comboBox1.DataSource = st.Tables["start"];          
            this.comboBox1.DisplayMember = "setout";
            this.comboBox1.ValueMember = "id";

select distinct(setout) as setout,id from planeinformation