Is there a way to apply a moving limit in SQL> - sql

Question

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

Answer 1

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

Answer 2

You left a lot of room for options with your statement:
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.
So, I'm going to leave the id out of it and first propose going with average of obs for each group as the simplest and probably the most practical, though maybe not the fastest to be running stat functions vs. limit:
MyModel.group(:day).average(:obs)
If you wanted the minimum:
MyModel.group(:day).minimum(:obs)
If you wanted the maximum:
MyModel.group(:day).maximum(:obs)
(Note: The following 2 examples are less efficient than just entering the SQL, but might be more portable.)
But you might want all three:
ActiveRecord::Base.connection.execute(MyModel.select('MIN(obs), AVG(obs), MAX(obs)').group(:day).to_sql).to_a
Or just the data without hashes:
ActiveRecord::Base.connection.exec_query(MyModel.select('MIN(obs), AVG(obs), MAX(obs)').group(:day).to_sql)
If you want median, see this question which is more DB specific, and there are other related posts about it if you search.
And for more, some DB's like postgres have variance(...), stddev(...), etc. built-in.
Finally, check out the query section in the Rails guide and ARel for more info on constructing queries. You can do a limit in an ActiveRecord relation via first or limit for example, and in ARel, take lets you do a limit. Subqueries are possible too, as shown in answers to this question, and so is group by, etc. If you are sharing this project with others, try to limit the amount of non-portable SQL you are using unless you plan on adding support for other databases on your own and maintaining that.