Syntax

ORDER BY { column-Name | ColumnPosition | Expression }
    [ ASC | DESC ]
    [ NULLS FIRST | NULLS LAST ]
    [ , column-Name | ColumnPosition | Expression 
    [ ASC | DESC ]
    [ NULLS FIRST | NULLS LAST ]
    ] * 

• order by 后面可以接列号（数字）、列名、别名、表达式、函数、分组函数
• order by 对空值的处理，DESC空值在前，ASC空值在后；
• order by子句中可以不含select中的列；
• 当使用select distinct或group by时，order by 不能使用select之外的列；

select job, avg(sal) "Average Salary" 
    from emp 
    group by job 
    order by "Average Salary" DESC;