窗口定义

window _ fun(tion cexpr)
OVER(
    PAR TITION BY ...  //对数据先进行分组
    ORDER BY ...   //分组之后的数据进行排序
    frame _ clause ... //它是进一步来限定窗口
)

这些选项的效果

CURRENT ROW它表示我们当前要处理的这一行数据
M PRECEDING表示当前行的往上M行
N FOLLDAING表示当前行的往下N行
UNBOUNDED PRECEDING表示分区的第一行
UNBOUNDED FOLLOWING表示分区的最后一行
示例：我们仍然使用销售表

select * from sales;

我们想要计算一下从数据累计到我当前它的一个总和

   select year,country,product,proft,
-> sum(profit) over (partition by country order by profit ros unbounded _ preceding) as runing _ total  //按照国家进行分区，按照利润进行排序
-> from sales
-> order by country. profit;

还可以用来计算从开始到当前数据的移动平均值

   select year,country,product,proft,
-> avg(profit) over (partition by country order by profit ros between 1 perceding and 1 following ) as runing _ avg 
-> from sales
-> order by country. profit;

窗口定义的另外一种写法

  select year,country,product,profit,
->first _ value(profit) over w as 'first' ,
->last _ value(profit) over w as 'last'
->from sales
->window w as (partition by country order by profit rows unbounded preceding)
->order by country,profit;