一、为什么要有窗口函数

我们直接用例子来说明，这里有一张学生考试成绩表

testScore

现在有个需求，需要查询的时候多出一列

subject_avg_score

传统方法肯定是用聚合，但是写起来很麻烦也很累赘，这时候窗口函数就排上了用场。

因为窗口函数不会像聚合一样将参与计算的行合并成一行输出，而是将计算出来的结果带回到了计算行上。

二、窗口函数的使用

1、聚合和窗口函数的区别

聚合：聚合函数(sum，min，avg……) + GROUP BY

窗口函数：聚合函数(sum，min，avg……) + OVER ( …… )

2、使用

还用上面的例子：

(1) 取每个不同科目的平均值

subject_avg_score

[这正是上面提到的需求]

这里的 OVER 里用到了

PARTITION BY

SELECT *,
avg("score") OVER (PARTITION BY "subject") as "subject_avg_score"
FROM "testScore"

(2) 取所有成绩的平均值

subject_avg_score

这里的 OVER 里为空

SELECT *,
avg("score") OVER () as "subject_avg_score"
FROM "testScore"

(3) 取此人该科目成绩班上排第几名

subject_rank_score

这里的 OVER 里用到了

PARTITION BY

ORDER BY

SELECT *,
avg("score") OVER (PARTITION BY "subject") as "subject_avg_score",
rank() OVER (PARTITION BY "subject" ORDER BY "score" DESC) as "subject_rank_score"
FROM "testScore"

(4) 提取 OVER 变量

如果在 sql 里写了很多重复的 OVER()，可以提取成一个 window 变量，简化代码。

SELECT *,
avg("score") OVER window_frame as "subject_avg_score",
avg("score") OVER window_frame as "subject_avg_score_2",
avg("score") OVER window_frame as "subject_avg_score_3"
FROM "testScore"
window window_frame as (PARTITION BY "subject")

参考资料

官方文档：http://www.postgres.cn/docs/9.3/tutorial-window.html

Postgresql窗口函数（一）

Postgresql窗口函数（二）