public void updateNum(int bookId,int saleNum){
		
		String querry = "select * from book where id="+bookId;
		String sql;
		
		DBManager dbm = null;
		Connection conn = null;
		Statement stmt = null;	
		try{
			ResultSet rs = null;
			dbm = new DBManager();
			conn = dbm.getConnection();
			stmt = conn.createStatement();
			rs = stmt.executeQuery(querry);
			int num=0;
			if(rs.next()){
				
			 num = rs.getInt(10);
			
			}
			sql= "update book set remainNum ="+(num-saleNum)+" where id="+ bookId+ ";";
			stmt.executeUpdate(sql);
		}catch(Exception e){
			e.printStackTrace()
			;
		}finally{
			dbm.closeAll(null, stmt, conn);
		}
		
	}