如何在PostgreSQL数据库中正确汇总日期？ [英] How correctly aggregate date in PostgreSQL database?

问题描述

我在 PostgreSQL 数据库中有一个表。

I have table in PostgreSQL database.

下表显示了城市中每个地下车站的火车每小时速度英格兰：

The table below shows you the hourly speed of trains in each underground station of the cities of England:

DATE_KEY            | STATION    | CITY      | SPEED
-------------------------------------------------------
2018-10-01 00:00:00 | Arsenal    | London    | 1078.125
2018-10-01 01:00:00 | Arsenal    | London    | 877.222
2018-10-01 02:00:00 | Arsenal    | London    | 1127.752
2018-10-01 00:00:00 | Beckton    | London    | 2866.375
2018-10-01 01:00:00 | Beckton    | London    | 1524.375
2018-10-01 02:00:00 | Beckton    | London    | 1618.533
2018-10-01 00:00:00 | Chesham    | Liverpool | 1567.588
2018-10-01 01:00:00 | Chesham    | Liverpool | 792.333
2018-10-01 02:00:00 | Chesham    | Liverpool | 1138.857
2018-10-01 00:00:00 | Farringdon | Liverpool | 1543.625
2018-10-01 01:00:00 | Farringdon | Liverpool | 538.666
2018-10-01 02:00:00 | Farringdon | Liverpool | 1587.583

我正在尝试获取这样的汇总数据：

I'm trying to get aggregated data like this:

DATE_KEY            | CITY      | AVG_SPEED
----------------------------------------------------
2018-10-01 00:00:00 | London    | 852.125
2018-10-01 01:00:00 | London    | 750.222
2018-10-01 02:00:00 | London    | 625.752
2018-10-01 00:00:00 | Liverpool | 804.588
2018-10-01 01:00:00 | Liverpool | 792.333
2018-10-01 02:00:00 | Liverpool | 952.857

换句话说，我需要计算城市中火车平均时速（AVG）

In other words, I need in result the hourly average (AVG) of trains speed in the city.

推荐答案

我认为您所需要的只是将AVG函数与group by子句一起使用：

I think all you need is to use AVG function with group by clause like:

SELECT  DATE_KEY, CITY, AVG(SPEED) as AVG_SPEED
FROM table
GROUP BY DATE_KEY, CITY

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