Operand should contain 1 column(s)

при выполнении запроса возникает ошибка Operand should contain 1 column(s), в чем может быть дело?

public List<Comment> read (int id) throws SQLException {
    Statement statement = null;

    List<Comment> commentList = new LinkedList<>();
    String sql = "SELECT (user_name, comment_content) FROM comments WHERE poster_id = '"+id+"'  ";

    try{
        statement = connection.createStatement();
        ResultSet resultSet = statement.executeQuery(sql);

        while (resultSet.next()){
            Comment comment = new Comment();
            comment.setUserName(resultSet.getString(1));
            comment.setContent(resultSet.getString(2));
            commentList.add(comment);
        }

    } catch (SQLException e) {
        throw new SQLException(e);
    }
    return commentList;
}