1.desc是descend 降序意思 ，asc 是ascend 升序意思，
2.与order by 一起用
/*在分数表里查询答案和分数两列，按分数从大到小排列（降序）*/
select answer, scores from assess_score order by scores desc
/*在分数表里查询答案和分数两列，按分数从小到大排列（升序）*/
select answer, scores from assess_score order by scores asc
/*在分数表里查询答案和分数两列，不加排序默认按分数从小到大排列（升序） ==asc
select answer, scores from assess_score order by scores
复合排序，先按列n1升序排列，再按n2降序排列
注意：如果采用多个字段排序，如果根据第一个字段排序重复了，会根据第二个字段排序；
