从sql数据库列获取最大值时未找到java列异常

我总是得到一个异常运行这个方法。它应该获得数据库表中列的最大值。这是我的密码：

private void genConNum(){
        try {
            String sql = "select max(contrno) from cis";
            PreparedStatement pst = conn.prepareStatement(sql);
            ResultSet rs = pst.executeQuery(sql);
            while(rs.next()){
                cont = rs.getString("contrno");
                num = Integer.parseInt(cont);
                num = num + 1;
                cont = String.valueOf(num);
                ContractNo.setText(cont);
            }
        } catch (SQLException ex) {
            JOptionPane.showMessageDialog(null, "Failed to generate Contract Number\n"+ex);
            ex.printStackTrace();
        }

    }

这是stacktrace

java.sql.SQLException: Column 'contrno' not found.
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:910)
at com.mysql.jdbc.ResultSet.findColumn(ResultSet.java:987)
at com.mysql.jdbc.ResultSet.getString(ResultSet.java:5584)
at client.AddCon.genConNum(AddCon.java:85)
at client.AddCon.access$1300(AddCon.java:20)
at client.AddCon$4.actionPerformed(AddCon.java:286)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6505)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
at java.awt.Component.processEvent(Component.java:6270)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4861)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
at java.awt.Container.dispatchEventImpl(Container.java:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:694)
at java.awt.EventQueue$3.run(EventQueue.java:692)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:708)
at java.awt.EventQueue$4.run(EventQueue.java:706)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

但该列存在于数据库本身中。我试图用相同的语句查询数据库，但在dbms中它可以工作。我想知道为什么它在java中不起作用。

将sql更改为：

String sql = "select max(contrno) contrno from cis";

将sql更改为：

String sql = "select max(contrno) contrno from cis";

---

按照@Grisha的建议更改sql。如果不想更改sql，则

---

更改

cont=rs.getString（“contno”）到
cont=rs.getString（“max（contno）”
按照@Grisha的建议更改sql。如果不想更改sql，则

更改
cont=rs.getString（“contno”）到
cont=rs.getString（“max（contno）”
可以通过这样做简化整个过程：

private void genConNum(){
        try {
            String sql = "select (max(contrno)+1) as max_val from cis";
            PreparedStatement pst = conn.prepareStatement(sql);
            ResultSet rs = pst.executeQuery(sql);
            while(rs.next()){
                ContractNo.setText(rs.getString("max_val"));
            }  
        } catch (SQLException ex) {
            JOptionPane.showMessageDialog(null, "Failed to generate Contract Number\n"+ex);
            ex.printStackTrace();
        }

    }

我将查询更改为为为您执行添加步骤，因此您所要做的就是将值提取为
字符串
，并将其设置到文本字段中。
这样做可以简化整个过程：

private void genConNum(){
        try {
            String sql = "select (max(contrno)+1) as max_val from cis";
            PreparedStatement pst = conn.prepareStatement(sql);
            ResultSet rs = pst.executeQuery(sql);
            while(rs.next()){
                ContractNo.setText(rs.getString("max_val"));
            }  
        } catch (SQLException ex) {
            JOptionPane.showMessageDialog(null, "Failed to generate Contract Number\n"+ex);
            ex.printStackTrace();
        }

    }

我更改了查询来为您执行添加步骤，因此您所要做的就是将值作为
字符串拉出，并在文本字段中进行设置。
此外，您可以使用适当的别名（即
String sql=“select max（contno）as max_contno from cis”
），然后使用
cont=rs.getInt（“max_contno”）而不是手动解析字符串。此外，您可以使用适当的别名（即
string sql=“从cis中选择max（contno）作为max_contno”
），然后使用
cont=rs.getInt（“max_contno”）而不是手动解析字符串。