How correctly aggregate date in PostgreSQL database?

问题

I have table in PostgreSQL database.

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

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

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

回答1:

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

回答2:

I know the dataset mentioned in the question is hourly values, if you want to compute the average across different timestamps (like "2018-10-01 02:45:08") then you can average speeds for each hour like this -

select DATE_TRUNC('day', a.DATE_KEY)+cast(DATE_PART('hour',a.DATE_KEY) as Integer)/1*INTERVAL '1 hour' as hour_key, city, avg(speed) as avg_speed 
    FROM table1 a group by 1, 2;

来源：https://stackoverflow.com/questions/52990917/how-correctly-aggregate-date-in-postgresql-database

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sql

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postgresql-10