Order by的几种用法

1. 按列名排序，默认是升序的。
　　 单列升序：select from order by ;
　　 单列降序：select from table_name order by desc;
　　 多列升序：select , from table_name order by , ;
　　 多列降序：select , from order by desc, desc;
　　 多列混合降序：select column_one, column_two from table_name order by column_one desc, column_two desc;
　　 2. 按列的位序排序, 默认是升序
　　 select * from order by 1
　　 3. NULL排序
　　 空值在前：select from order by NULLS FIRST;
　　 空值在后：select from order by NULLS LAST;