On Oct 2, 2007, at 9:29 , Stefan Schwarzer wrote:
> How would I do that? I really have no clue...
The key is to build it up in steps.
select id_country, year, var_1, val_1, var_2, val_2
-- Second step:
-- value for year for each country of var_1
from (select id_country, year, id_variable as var_1, "value" as val_1
from my_table) as val_1
-- value for year for each country for var_2
natural join (select id_country, year, id_variable as var_2, "value"
as val_2
from my_table) as val_2
-- First step
-- for each country, find the minimum common year (which the join
will do) for the two
-- variables you're interested in (var_1 and var_2).
natural join (select id_country, var_1, var_2, min(year) as year
from (select id_country, year, id_variable as var_1
from my_table) as var_1
natural join (select id_country, year, id_variable
as var_2
from my_table) as var_2
group by id_country, var_1, var_2) as min_common_year
where id_country = :id_country
and var_1 = :var_1
and var_2 = :var_2;
Check your explain analyze output: if the planner doesn't push up
the :var_1, :var_2, and :id_country_id values up into subqueries, you
might want to add them as where clauses.
As an aside, I assume you've rewritten the table column names: if you
haven't, as it's an SQL keyword, "value" is a particularly poor
choice of column name. I'd probably rename "year" as well.
Hope this helps.
Michael Glaesemann
grzm seespotcode net
