Postgres select all columns but group by one column

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有刺的猬
有刺的猬 2021-01-31 11:43

I have a simple table with a unit_id oid, time timestamp, diag bytea. The primary key is a combination of both time and unit_id.

The idea behind this query is to get the

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  • 2021-01-31 12:29

    You can join the grouped select with the original table:

    SELECT d.time, d.diag, d.unit_id
    FROM(
        SELECT unit_id, max(time) as max_time
        FROM diagnostics.unit_diag_history
        GROUP BY unit_id
    ) s JOIN diagnostics.unit_diag_history d
    ON s.unit_id = d.unit_id AND s.max_time = d.time
    
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  • 2021-01-31 12:32

    Any time you start thinking that you want a localized GROUP BY you should start thinking about window functions instead.

    I think you're after something like this:

    select unit_id, time, diag
    from (
        select unit_id, time, diag,
               rank() over (partition by unit_id order by time desc) as rank
        from diagnostics.unit_diag_history
    ) as dt
    where rank = 1
    

    You might want to add something to the ORDER BY to consistently break ties as well but that wouldn't alter the overall technique.

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