如何将 sql 数据库中的列总和显示到 Java 应用程序中？

Question

我有一个存储项目记录的数据库，在名为 'quantity' 的列之一中，我希望能够检索此 'quantity' 金额并显示在我的 java 申请。如果您在下面看到我的代码，您会发现我正在尝试从我的数据库中打印出该金额，但它不起作用。当我运行我的应用程序时，它总是运行ning 而不是尝试。我不确定我是否没有正确执行 SQL 代码，但这只是我的猜测，如果您知道如何解决这个问题，请告诉我，谢谢。

private void initComponents() {
private String sum;


try {
            Class.forName("com.mysql.jdbc.Driver");
            Connection con1 = DriverManager.getConnection("jdbc:mysql://localhost/restock", "root", "password");
            String template = "SELECT SUM(quantity) as sumquantity FROM buysupply WHERE itemtype IN ('Disposable mask', 'Clothed mask', 'N95 mask')";
            PreparedStatement pst = con1.prepareStatement(template);
            ResultSet rs = pst.executeQuery();
            sum = rs.getString("sumquantity");
            System.out.print("############################" + sum);
                  
        } catch (ClassNotFoundException ex) {
            Logger.getLogger(stock.class.getName()).log(Level.SEVERE, null, ex);
        } catch (SQLException ex) {
            Logger.getLogger(stock.class.getName()).log(Level.SEVERE, null, ex);
        }




}

供您参考，错误是这样的：

Loading class `com.mysql.jdbc.Driver'. This is deprecated. The new driver class is `com.mysql.cj.jdbc.Driver'. The driver is automatically registered via the SPI and manual loading of the driver class is generally unnecessary.
Aug 06, 2020 1:22:03 AM pleasework.stock initComponents
SEVERE: null
java.sql.SQLException: Before start of result set
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:129)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:97)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:89)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:63)
    at com.mysql.cj.jdbc.result.ResultSetImpl.checkRowPos(ResultSetImpl.java:492)
    at com.mysql.cj.jdbc.result.ResultSetImpl.getString(ResultSetImpl.java:845)
    at com.mysql.cj.jdbc.result.ResultSetImpl.getString(ResultSetImpl.java:863)
    at pleasework.stock.initComponents(stock.java:92)
    at pleasework.stock.<init>(stock.java:30)
    at pleasework.stock.run(stock.java:823)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
    at java.awt.EventQueue.access0(EventQueue.java:97)
    at java.awt.EventQueue.run(EventQueue.java:709)
    at java.awt.EventQueue.run(EventQueue.java:703)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:74)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:205)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

Answer 1

您必须先调用 rs.next() 将第一行加载到结果集中，然后才能使用 rs.getString('...')。

ResultSet rs = pst.executeQuery();
rs.next();
sum = rs.getString("sumquantity");
System.out.print("############################" + sum);

您还应该使用 rs.next() 检查您是否收到了结果。例如：

ResultSet rs = pst.executeQuery();
if (rs.next()) {
    sum = rs.getString("sumquantity");
    System.out.print("############################" + sum);
} else {
    System.out.print("Query didn't return any results");
}

另一件引起注意的事情：您使用 getString() 来获取数值。您可能想改用 rs.getInt() 或 rs.getDouble()。

如何将 sql 数据库中的列总和显示到 Java 应用程序中？

How to display sum of columns from sql database into Java application?

java

mysql

sql

swing

netbeans