Question

I have the following work on my application, in which I am trying to update the value total on my mysql database table called “porcobrar2012”. However, the only value that gets updated is the last one generated in the while loop. Why? all values are been printout on the screen with no problem, but those values are not getting updated in the database.
Here is the code:

BigDecimal total = new BigDecimal("0"); 
  try
{
        //Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
        //Connection connection=DriverManager.getConnection("jdbc:odbc:db1","","");
        Class.forName("com.mysql.jdbc.Driver").newInstance();
        Connection connection=DriverManager.getConnection("jdbc:mysql://localhost/etoolsco_VecinetSM?user=etoolsco&password=g7Xm2heD41");
        Statement statement=connection.createStatement();
        String query;
        query="SELECT * FROM porcobrar2012"; 
        ResultSet resultSet=statement.executeQuery(query);
    while(resultSet.next())
    {
    out.println(resultSet.getString(2)+"");out.println(resultSet.getBigDecimal(3)+"");out.println(resultSet.getBigDecimal(4)+"");out.println(resultSet.getBigDecimal(5)+"");out.println(resultSet.getBigDecimal(6)+"");out.println(resultSet.getBigDecimal(7)+"");out.println(resultSet.getBigDecimal(8)+"");out.println(resultSet.getBigDecimal(9)+"");out.println(resultSet.getBigDecimal(10)+"");out.println(resultSet.getBigDecimal(11)+"");out.println(resultSet.getBigDecimal(12)+"");out.println(resultSet.getBigDecimal(13)+"")out.println(resultSet.getBigDecimal(14)+"");out.println(resultSet.getBigDecimal(15)+"");
    total = resultSet.getBigDecimal(3).add(resultSet.getBigDecimal(4)).add(resultSet.getBigDecimal(5)).add(resultSet.getBigDecimal(6)).add(resultSet.getBigDecimal(7)).add(resultSet.getBigDecimal(8)).add(resultSet.getBigDecimal(9)).add(resultSet.getBigDecimal(10)).add(resultSet.getBigDecimal(11)).add(resultSet.getBigDecimal(12)).add(resultSet.getBigDecimal(13)).add(resultSet.getBigDecimal(14)).add(resultSet.getBigDecimal(15));
    String query1;
    query1="UPDATE porcobrar2012 SET total=total";
    PreparedStatement ps = connection.prepareStatement(query1);
    ps.executeUpdate();

    out.println(total);

}

    connection.close();
statement.close();




} 

  catch (Exception e) 
{
      //e.printStackTrace();
      out.println(e.toString());
}

Answer 1

It’s because the update closes the existing result set. But I would ask why you aren’t doing the addition in a single UPDATE statement without any prior query, at the database, no loops, no BigDecimals. Rule one of database programming is ‘don’t move the data further than you need to’. It would be many times as efficient to just write “UPDATE porcobrar2012 SET a=b+c+d+…”. And you can remove the Class.forName() call too: it hasn’t been required for years.