用Java

Question

我需要动态更新数据库信息。我用过MySQL。这是我的代码：

static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";
static final String DB_URL = "jdbc:mysql://localhost/EMP";

static final String USER = "root";
static final String PASS = "";

/**
 * @param args
 */
/**
 * @param args
 */
public static void main(String[] args) {
    Connection con = null;
    PreparedStatement pstmt = null;

    try{
        Class.forName("com.mysql.jdbc.Driver");

        System.out.println("Connecting to database....");
        con = DriverManager.getConnection(DB_URL,USER,PASS);

        System.out.println("Creating statement...");

        String psql;

        psql = "Update Employees SET age = ? WHERE id = ?";

        pstmt = con.prepareStatement(psql);
        pstmt.setInt(1, 20);
        pstmt.setInt(2, 100);

        int prows = pstmt.executeUpdate(psql);
        System.out.println("PRows impacted : " + prows);

        psql = "SELECT id, first, last, age FROM Employees";             
        ResultSet rs = pstmt.executeQuery(psql);

        while(rs.next()){
            int id = rs.getInt("id");
            int age = rs.getInt("age");
            String first = rs.getString("first");
            String last = rs.getString("last");

            System.out.print("ID: " + id);
            System.out.print(", Age: " + age);
            System.out.print(", First: " + first);
            System.out.println(", Last: " + last);
        }   
        rs.close();
        pstmt.close();
        con.close();
    } catch(SQLException se){
        se.printStackTrace();
    } catch(Exception e){
        e.printStackTrace();
    } finally{
        try{
            if(pstmt != null)
                pstmt.close();
        } catch (SQLException se2){
        }
        try{
            if(con != null)
                con.close();
        } catch (SQLException se){
            se.printStackTrace();
        }
    }
    System.out.println("Goodbye!");
}

但我得到的SQL语法错误如下：

com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error 
in your SQL syntax; check the manual that corresponds to your MySQL server 
version for the right syntax to use near '? WHERE id = ?' at line 1
  at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
  at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
  at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
  at java.lang.reflect.Constructor.newInstance(Unknown Source)
  at com.mysql.jdbc.Util.handleNewInstance(Util.java:425)
  at com.mysql.jdbc.Util.getInstance(Util.java:408)
  at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:943)
  at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3973)
  at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3909)
  at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2527)
  at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2680)
  at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2486)
  at com.mysql.jdbc.StatementImpl.executeUpdateInternal(StatementImpl.java:1552)
  at com.mysql.jdbc.StatementImpl.executeLargeUpdate(StatementImpl.java:2607)
  at com.mysql.jdbc.StatementImpl.executeUpdate(StatementImpl.java:1480)
  at Practice2.main(Practice2.java:39)

我已经google了解决方案。但没有得到任何结果。这有什么不对？