Re: SELECT only those values of table B which have a value in table A

On Thu, 25 Feb 2010 09:34:19 +0100, Stefan Schwarzer wrote:
> Hi there,
>
> I have tables with values for each country of the world.
>
> Now, there is the possibility for the user to generate regional
> (Europe, Africa...) or subregional (Western Europe, Central
> Europe...) aggregations on-the-fly. In some cases, these
> aggregations need to be calculated by using the population of each
> region too, as a denominator, which looks like this:
>
>
>         SELECT
>             COALESCE(r.name, '''') AS name,
>             d.year_start AS year,
>             SUM(d.value * d_nom.value) / SUM(d_nom.value) AS value
>         FROM
>             pop_total_access_water AS d
>         RIGHT JOIN
>             countries_view AS c ON c.id = d.id_country
>         RIGHT JOIN
>             pop_total AS d_nom ON d_nom.id_country = d.id_country
>         RIGHT JOIN
>             subregions_view AS r ON r.id = c.sreg_id
>         WHERE
>             (d.year_start = 1990 ) AND
>             (d_nom.year_start = 1990 ) AND
>             d_nom.value <> 0
>         GROUP BY
>             r.name, d.year_start
>         ORDER BY 1,2
>
> Now, I would like to use from table "d_nom" only these countries
> which have a value in table "d" as well.

Without thinking much (so this may be completely wrong for some reason/additional
changes may be needed), why do you use a right join to the d_nom table?
An ordinary inner join should give you only these countries which have
their rows in the d table.

--
Michał Politowski
Talking has been known to lead to communication if practiced carelessly.