Re: Complicated "group by" question

Andrew Perrin wrote:
> I have a table of people ("reviewers"), a table of review assignments
> ("assign"), and a table of review acceptances ("accept"). I would like to
> be able to write a query to return the latest (e.g., max(assign_date))
> assignment for each reviewer, plus the acc_id field from "accept".  I
> think I should be able to do this with a GROUP BY clause, but am having no
> luck.
> 
> Table structure:
> 
> reviewers    assign        accept
> -----------------------------------------
> reviewer_id    assign_id    accept_id
>         reviewer_id    assign_id
> ...        assign_date
>         ...        ...
> 


I think you want to write a non-GROUPed query using "DISTINCT ON". 
Something like this:

SELECT DISTINCT ON (reviewer_id,assign_id) reviewer_id, assign_id, assign_date, accept_id
FROM reviewers
JOIN assign USING (reviewer_id)
JOIN accept USING (accept_id)
ORDER BY reviewer_id, assign_id, assign_date DESC;


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