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

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. What happens now is that, if my table "d" has for example 2 countries with values and 2 countries without values, but table d_nom has for all4 countries values, my end result would be only 50% of the real value, which is wrong. I have to multiply and to divide through the same amount of countries.

Could anyone give me a hint how this would work? Do I need to add a subquery? 

Thanks a lot!

Stef