如何在java应用程序中显示sql数据库中列的总和？

tkqqtvp1 于 2021-07-24 发布在 Java

关注(0)|答案(1)|浏览(308)

我有一个数据库，它将我的项目的记录存储在一个名为'quantity'的列中，我希望能够检索这个'quantity'数量并显示在我的java应用程序中。如果你看到我下面的代码，你可以看到我正试图打印出从我的数据库，但它是不起作用的金额。当我运行应用程序时，运行的总是捕获，而不是尝试。我不确定是否可能我没有做的sql代码正确，但这只是我的猜测，如果你知道如何解决这个问题，请让我知道谢谢你。

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);
        }

}

关于您的信息，错误如下：

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$16.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.access$500(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:709)
    at java.awt.EventQueue$3.run(EventQueue.java:703)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$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)

Java sql mysql swing netbeans

来源：https://stackoverflow.com/questions/63270349/how-to-display-sum-of-columns-from-sql-database-into-java-application

1条答案

按热度按时间

kwvwclae1#

你得打电话 rs.next() 在使用之前将第一行加载到结果集中 rs.getString('...') .

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

你也应该使用 rs.next() 检查是否收到结果。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");
}

另一个吸引注意力的东西：你用 getString() 获取数值。你可能想用 rs.getInt() 或者 rs.getDouble() 相反。

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如何在java应用程序中显示sql数据库中列的总和？

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