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I have a parent row and then many child rows, and I must aggregate statistics from these child rows.

For a more concrete example let's imagine I have one round of golf (parent row) and then child rows with the score of each hole. I need to aggregate statistics on these rows, like number of birdies, pars, bogies, putts and so forth. And then put these numbers in a round_detail table.

What would be the best solution to this?

I'm using postgres if it matters, but this is probably just a general sql propblem.

EDIT: As requested!1/6549c

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Please post the CREATE TABLE for the tables involved and some sample data. Ideally as a runnable exampel on – a_horse_with_no_name Feb 7 '13 at 16:11
Have edited with sql statements – Iceman Feb 7 '13 at 18:36
I want to clarify that I know how to group by and all that stuff, what I'm looking for is a best practice solution to do this efficiently. It could be done with a trigger, that updates a round_detail table with the necessary stats, or maybe a stored proc. Alternative solutions, like using mongodb for the stats are also welcome. But it must run on Linux. – Iceman Feb 7 '13 at 18:38
So you aren't really looking for a query that returns the results? – a_horse_with_no_name Feb 7 '13 at 18:42
Not necessarily, it could be done that way or done with a trigger. I'm just trying to figure out what would be the best way. I thought doing it just once would provide better performance later, because rounds are rarely edited afterwards. – Iceman Feb 7 '13 at 18:45
up vote 4 down vote accepted

The concept you consider is known as materialized view. Since you comment:

rounds are rarely edited afterwards

.. I think this should be good idea for your case. Some RDBMS' have distinct objects for that. In Postgres it's just a matter of writing the results from a view or SRF (set returning function) to a table. You can find a basic code example for refreshing MVs in the manual here.

Refer to this related answer on SO for the more sophisticated regime I use for that purpose.

Your query to (re-)create a materialized view could look something like this:

SELECT AS course
      ,COUNT(NULLIF(s.strokes = h.par, FALSE))     AS pars
      ,COUNT(NULLIF(s.strokes = h.par - 1, FALSE)) AS birdies
      -- more columns ..
FROM   score  s
JOIN   round  r ON = s.round_id
JOIN   course c ON = r.course_id
JOIN   hole   h ON = s.hole_id
GROUP  BY 1,2,3

I simplified your test case and added the query: ->sqlfiddle.

share|improve this answer
I wish SQL Server had a true Boolean data type like this instead of just Boolean-like bit values! So you could for example do WHERE IsNumeric(a) instead of WHERE IsNumeric(a) = 1. – ErikE Feb 9 '13 at 6:29
Thanks for this great answer. – Iceman Feb 9 '13 at 19:51
Have been playing around with this a bit, and this is great stuff, thanks again. – Iceman Feb 9 '13 at 20:55

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