SQL语言的默认排序方式是升序，ORDER BY 子句若未显式指定升序(ASC)或降序(DESC)，那么就认按默认升序排序。例如下列语句为默认按升序排序select * from t1 order by col1;它与下列语句等效select * from t1 order by col1 asc;
默认是升序，当order by A，B。以A升序为主，再以B升序进行排序
