code: private String queryPlaceholder(int length) {
          StringBuilder sb = new StringBuilder(length * 2 + 1);
          sb.append("(?");
          for (int i = 1; i < length; i++) {
              sb.append(",?");
          }
          sb.append(")");
          return sb.toString();
      }
docs: Create a placeholder for SQL query syntax e.g. "(?,?,?)".
  
  @param length Number of values that will be inserted to the placeholder.
  @return A placeholder for SQL query syntax.