Question

If I remove beforeFirst() my function does only first record in ResultSet and go to end. If I use beforeFirst() I get an error ResultSet IS_TYPE_FORWARD_ONLY.

try {
    ResultSet rs = stat.executeQuery("select _id, godziny_id from tblZmiany where harmonogram_id = " + h_id + " order by pracownik_id, Dzien");
    rs.beforeFirst();
    while (rs.next()) {
        if (stat.executeUpdate("insert into tblWykonanie (Zmiana_id, Godziny_id) values ('" + rs.getLong(1) + "', " + rs.getInt(2) + ");") < 1) {
            // Jeśli insert nie wstawił kolejnego rekordu
            error_code = "Wystąpił problem podczas zatwierdzania harmonogramu.";
            return false;
        }
    }
} catch (SQLException e) {
    e.printStackTrace();
    System.out.println(e.getMessage());
    return false;
} catch (Exception e) {
    error_code = e.getMessage();
    return false;
}

return true;

Answer 1

You’re using the same Statement object for both the SELECT and the INSERT. From the Javadoc:

A ResultSet object is automatically closed when the Statement object that generated it is closed, re-executed, or used to retrieve the next result from a sequence of multiple results.

To fix the problem, use a separate Statement object for the executeUpdate() call.

Also, I strongly advise you to change the code to use PreparedStatement with bound arguments (represented by ?). Building SQL statements bit by bit as you’re doing right now could open up security vulnerabilities.