Re: Find min year and min value - Mailing list pgsql-general
From | Michael Glaesemann |
---|---|
Subject | Re: Find min year and min value |
Date | |
Msg-id | 49C9F9A7-F165-4702-96E0-FB35EAC79F03@seespotcode.net Whole thread Raw |
In response to | Re: Find min year and min value (Steve Crawford <scrawford@pinpointresearch.com>) |
Responses |
Re: Find min year and min value
(Stefan Schwarzer <stefan.schwarzer@grid.unep.ch>)
Re: Find min year and min value (Stefan Schwarzer <stefan.schwarzer@grid.unep.ch>) |
List | pgsql-general |
On Oct 2, 2007, at 11:10 , Steve Crawford wrote: > As others have noted, the query *can* be written. But it appears to me > that you are struggling against your table layout. The current schema he has is commonly called EAV (entity-attribute- value) and is generally frowned upon. Now, in his particular case it may be justified if the "value" column values are actually all of the same type, such as currency amounts for each category. If this is the case, I suggest renaming the column to be more descriptive of what is actually stored: likewise the id_variable column. > Before struggling > with ever more complicated queries, I'd consider restructuring your > table(s). There are many possibilities depending on the current nature > of your data, how you expect it to change and the queries you > expect to > run against it. For example: > country_id > data_year > gdp > fish_catch This would be one way to do it. However, each time you add a new category you'd need to add a new column to the table: not very flexible. You can also have the same functionality by adding a new table for each category: > Alternately, you could have a gdp table and a fish_catch table which > would be easily joined to give the same result. Expanding on this: create table fish_catches (country text not null, data_year date not null, primary key (country, data_year), fish_catch numeric not null); create table gdp (country text not null reference countries data_year date not null, primary key (country, data_year), gdp numeric not null); This makes your queries quite simple: select country, data_year, fish_catch, gdp from fish_catches natural join gdp where country = :country order by data_year limit 1; or select country, data_year, fish_catch, gdp from fish_catches natural join gdp natural join (select country, min(data_year) as data_year from gdp natural join fish_catch group by country) min_data_year where country = :country; Splitting categories into separate tables also eliminates the necessity of worrying about NULL, which can lead to unexpected behavior if you aren't careful. Michael Glaesemann grzm seespotcode net
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