数据库查询

利用Connection对象的createStatement方法建立Statement对象，利用Statement对象的executeQuery()方法执行SQL查询语句进行查询，返回结果集，再形如getXXX()的方法从结果集中读取数据。经过这样的一系列步骤就能实现对数据库的查询。

【例】Java应用程序访问数据库。应用程序打开考生信息表ksInfo，从中取出考生的各项信息。设考生信息数据库的结构如下：

import java.awt.*; import java.awt.event.*; import javax.swing.*; import java.util.*; import java.sql.*; public class Example10_9 extends JFrame implements ActionListener{ public static Connection connectByJdbcodbc(String url, String username,String password){ Connection con = null; try{ Class.forName("sun.jdbc.odbc.JdbcOdbcDriver"); //加载ODBC驱动程序 } catch (Exception e){ e.printStackTrace(); return null; //加载失败，连接不成功 } try{ con = DriverManager.getConnection(url, username, password); } catch (SQLException e){ e.printStackTrace(); return null; //连接失败 } return con; //连接成功 } String title[] ={"考号", "姓名", "成绩", "地址", "简历"}; JTextField txtNo = new JTextField(8); JTextField txtName = new JTextField(10); JTextField txtScore = new JTextField(3); JTextField txtAddr = new JTextField(30); JTextArea txtresume = new JTextArea(); JButton prev = new JButton("前一个"); JButton next = new JButton("后一个"); JButton first = new JButton("第一个"); JButton last = new JButton("最后一个"); Statement sql; //SQL语句对象 ResultSet rs; //存放查询结果对象 Example10_9(Connection connect){ super("考生信息查看窗口"); setSize(450, 350); try{ sql = connect.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_READ_ONLY); rs = sql.executeQuery("SELECT * FROM ksInfo"); Container con = getContentPane(); con.setLayout(new BorderLayout(0, 6); JPanel p[] = new JPanel[4]; for (int i = 0; i < 4; i++){ p[i] = new JPane(new FlowLayout(FlowLayout.LEFT, 8, 0)); p[i].add(new JLabel(title[i])); } p[0].add(txtNo); p[1].add(txtName); p[2].add(txtScore); p[3].add(txtAddr); JPanel p1 = new JPane(new GridLayout94, 1, 0, 8)); JScrollPane jsp = new JScrollPane(txtResume, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_NEVER); jsp.setPreforredSize(new Dimension(300, 60); for (int i = 0; i < 4; i++){ p1.add(p[i]); } JPanel p2 = new JPanel(new FlowLayout(FlowLayout.LEFT, 10, 0); p2.add(new JLabel(title[4])); p2.add(jsp); Jpanel p3 = new Jpanel(); p3.add(prev); p3.add(next); p3.add(first); p3.add(last); prev.addActionListener(this); next.addActionListener(this); first.addActionListener(this); last.addActionlistener(this); rs.first(); readRecord(); } catch (Exception e){ e.printStackTrace(): } setVisible(ture); } public void modifyRecord(Connection connect){ String stuNo = (String)JOptionPane.showInputDialog(null, "请输入考生考号", "输入考号对话框", JOptionPane.PLAIN_MESSAGE, null, null, ""); try { sql = connect.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_READ_ONLY); rs = sql.executeQuery("SELECT * FROM ksInfo"); Container con = getContentPane(); con.setLayout(new Boarderlayout(0, 6)); Jpanel p[] = new JPanel[4]; for (int i = 0; i < ; i++){ p[i] = new JPane(new FlowLayout(flowLayout.LEFT, 8, 0)); p[i].add(new JLabel(title[i])); } p[0].add(txtNo); p[1].add(txtName); p[2].add(txtScore); p[3].add(txtAddr); Jpanel p1 = new Jpane(new GridLayout(4, 1, 0, 8)); JScrollPane jsp = new JScrollPane(txtResume, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_NEVER); jsp.setPreferredSize (new dimension(300, 60)); for (int i = 0; i < 4; i++){ p1.add(p[i]); } Jpanel p2 = new JPanel(new FlowLayout(FlowLayout.LEFT, 10, 0)); p2.add(new JLableI(title[4])); p2.add(jsp); JPanel p3 = new JPanel(); p3.add(prev); p3.add(next); p3.add(first); p3.add(last); prev.addActionListener(this); next.addActionListener(this); first.addActionListenerIthis); last.addActionListener(this); rs.first(); readRecord(); } catch (Exception e){ e.printStackTrace(); } setVisible(true); } boolean readRecord(){ try{ txtNo.setText(rs.getString("考号")); txtName.setText(rs.getString("姓名")); txtScore.setText(rs.getString("成绩")); txtAddr.setText(rs.getString("地址")); txtResume.setText(rs.getString("简历")); } catch (SQLException e){ e.printStackTrace(); return false; } return true; } public void actionPerformed(ActionEvent e){ try{ if (e.getSource() == prev)rs.previous(); else if (e.getSource() == next)rs.next(); else if (e.getSource() == first)rs.first(); else if (e.getSource() == last)rs.last(); readRecord(); } catch (Exception e2){} } public static void main(String args[]){ connection connect = null; JFrame .setDefaultLookAndFeeDecorated(true); Font font = new Font("JFrame", Font.PLAIN, 14); if ((connect =connectByJdbcOdbc("jdbc:odbc:redsun", "xia", "1234")) == null){ JOptionPane.showMessageDialog(null, "数据库连接失败!"); System.exit ( - 1); } new Example10_9(connect); //创建对象 } }

Java数据库更新

数据库更新操作包括数据表创建、删除、以及数据表记录的增加、删除、修改等操作。如果利用数据 SQL命令实现，则利用Statement对旬的executeUpdate()方法，执行SQL的update语句，实现数据表的修改；执行SQL的insert语句，实现数据表记录的添加。

例如，在前面数据为查询例子基础上，再增加对数据表的修改和插入。限于篇幅，不再给出完整程序，只给出实现修改和插入的方法。程序可再增设插入、，，除保存按钮，通过已有的浏览，定位到数据表的特定位置，对痈疽记录进行编辑修改，或插入，或删除，然后按保存按钮，完成修改后的数据表保存。

下面用代码说明数据表更新的方法。与数据表连接时，需指定获得的ResultSet 对象是可更新的。

stmt = connect.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_UPDATABLE);