$aryType = $mType->field('id, add_time')->order('add_time DESC')->select();
echo $mType->getLastSql();
// 输出SELECT `id`,`add_time` FROM `x_article_type` ORDER BY add_time DESC

$aryType = $mType->field('id, add_time', true)->order('add_time DESC')->select();
SELECT `name`,`add_time` FROM `x_article_type` ORDER BY add_time DESC

add_time 前面只是多了一个空格...