Window functions in SQL

[MurphyL]

窗口函数（Window Function） 是 SQL2003 标准中定义的一项新特性，并在 SQL2011、SQL2016 中又加以完善，添加了若干处拓展。窗口函数不同于我们熟悉的普通函数和聚合函数，它为每行数据进行一次计算：输入多行（一个窗口）、返回一个值。

Window functions applies aggregate and ranking functions over a particular window (set of rows). OVER clause is used with window functions to define that window. OVER clause does two things :

• Partitions rows into form set of rows. (PARTITION BY clause is used)
• Orders rows within those partitions into a particular order. (ORDER BY clause is used)

window_function (expression) OVER (
   [ PARTITION BY part_list ]
   [ ORDER BY order_list ]
   [ { ROWS | RANGE } BETWEEN frame_start AND frame_end ] )

参考

• Window functions in SQL - GeeksforGeeks