code: private static String preparePlaceHolders(int length) {
          StringBuilder builder = new StringBuilder();
          for (int i = 0; i < length; ) {
              builder.append("?");
              if (++i < length) {
                  builder.append(",");
              }
          }
          return builder.toString();
      }
docs: Creates a list of placeholders for use in a PreparedStatement
  
  @param length number of placeholders
  @return String of placeholders, seperated by comma