从数据库中检索以显示为折线图

Question

我正在尝试显示我的数据库中的数据。我目前一次执行一个查询，以获得每年的一个特定结果。我需要对数据库中的所有列执行此操作，但这会使代码变得异常混乱。有没有更简单的方法可以从数据库中读取数据以显示为折线图？

当前代码sn-p：

DefaultCategoryDataset dataset = new DefaultCategoryDataset();

String sql = "SELECT SUM(all_motor_vehicles), Year FROM Vehicle WHERE Year = 2005";
pst = con.prepareStatement(sql);
rs = pst.executeQuery();
amount = Integer.parseInt(rs.getString(1)); //get the result
yearFromDB = rs.getString(2);
System.out.println(amount + " " + yearFromDB); //print result


int amount2006;
String yearFromDB2006;
String sql2 = "SELECT SUM(all_motor_vehicles), Year FROM Vehicle WHERE Year = 2006";
pst = con.prepareStatement(sql2);
rs = pst.executeQuery();
amount2006 = Integer.parseInt(rs.getString(1)); //get the result
yearFromDB2006 = rs.getString(2);
System.out.println(amount2006 + " " + yearFromDB2006); //print result

int amount2007;
String yearFromDB2007;
String sql3 = "SELECT SUM(all_motor_vehicles), Year FROM Vehicle WHERE Year = 2007";
pst = con.prepareStatement(sql3);
rs = pst.executeQuery();
amount2007 = Integer.parseInt(rs.getString(1)); //get the result
yearFromDB2007 = rs.getString(2);
System.out.println(amount2007 + " " + yearFromDB2007); //print result

dataset.addValue(amount, allMotorVehicles, yearFromDB);
dataset.addValue(amount2006, allMotorVehicles, yearFromDB2006);
dataset.addValue(amount2007, allMotorVehicles, yearFromDB2007);

JFreeChart chart = ChartFactory.createLineChart("Traffic by Vehicle type", "Vehicle", `"Amount",dataset);`

数据库中的信息片段：

实施建议的查询给出 SQL 错误：

在 DB 中执行查询返回：

我可以使用 rs.getString(1) 检索 2005 年，它是使用 rs.getString(2) 的相应数字，但是当我尝试检索 2006 年时，它是通过 rs.getString(3) 和 rs.getString( 4) 我得到一个 SQLException 错误

String sql = "SELECT Year, SUM(all_motor_vehicles) AS allmotor\n" +
          

"FROM Vehicle\n"
            + "WHERE Year IN (2005, 2006, 2007)\n"
            + "GROUP BY YEAR;";
    pst = con.prepareStatement(sql);
    rs = pst.executeQuery();

    amount = Integer.parseInt(rs.getString(2)); //retrieves amount
    yearFromDB = rs.getString(1);
    System.out.println(amount + " " + yearFromDB); //retrieves year (2005)
    
    System.out.println(rs.getString(3) + " " + rs.getString(4)); //gives me an SQL ERROR

Answer 1

您应该改为执行单个 GROUP BY 查询，在其中按年份汇总销售额：

SELECT Year, SUM(all_motor_vehicles) AS all_sales
FROM Vehicle
WHERE Year IN (2005, 2006, 2007)  -- or whatever years you want in your report
GROUP BY Year;

我不会费心为您提供上述查询的样板 JDBC 代码，因为您似乎已经掌握了这些基础知识。请注意，我为 SUM() 提供了一个别名，以防您想使用它访问 Java 中的结果集。