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Editorial Team
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Asked: 2026-06-19T03:48:28+00:00

I have a large database I use for plotting and data examination. For simplicity,

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I have a large database I use for plotting and data examination. For simplicity, say it looks something like this:

|    id    |    day    |    obs    |
+----------+-----------+-----------+
|    1     |    500    |    4.5    | 
|    2     |    500    |    4.4    | 
|    3     |    500    |    4.7    | 
|    4     |    500    |    4.8    | 
|    5     |    600    |    5.1    | 
|    6     |    600    |    5.2    | 
                ...

This could be stock market data, where we have many points per day that are measured.

What I want to do is look at much longer trends, where the multiple points per day are unnecessarily resolved, and clog my plotting application. (I want to look at 30000 days, each has about 100 observations).

Is there a way to do something like SELECT ... LIMIT 1 PER "day"

I suppose I could perform a few SELECT DISTINCT queries to find correct ID’s, but I’d rather do something simple if it is built in.

It doesn’t matter if its the first, last, or an average value per day. Just a single value. I just prefer what is fastest.

Also, this I’d like to do this for Postgres, MySQL, and SQLite. My application is built to use all three and I frequently switch between them.

Thanks!

Background: This is for a Ruby on Rails plotting application, so a trick with ActiveRecord will work too. https://github.com/ZachDischner/Rails-Plotter

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1 Answer

  1. Editorial Team

    You need to tag your question with the brand of RDBMS you’re using. Frequently for Rails developers, they’re using MySQL, but the answer to your question depends on this.

    For all brands except for MySQL, the correct and standard solution is to use windowing functions:

    SELECT * FROM (
  SELECT ROW_NUMBER() OVER (PARTITION BY day) AS RN, *
  FROM stockmarketdata
) AS t
WHERE t.RN = 1;

    For MySQL, which doesn’t support windowing functions yet, you can simulate them in a kind of clumsy way with session variables:

    SELECT * FROM (SELECT @day:=0, @r:=0) AS _init,
(
  SELECT IF(day=@day, @r:=@r+1, @r:=0) AS RN, @day:=day AS d, *
  FROM stockmarketdata
) AS t
WHERE t.RN = 1
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