Dividing count on one table from another and grouping the results

I am trying to calculate the ratio of rows in table A to rows in table B. This would have been easy if I only had to calculate the ratio for the whole sets, but I want to group them by a category that exists in table B. Table A and B are linked through a primary key (id).

Edit:

I tried the following:

select
    (select count(*) from class_a a left join football_players b on (b.id = a.id) group by age)
    /
    (select count(*) from football_players group by age)

but the group by command does not work here.

Solution

You need an outer join, then divide count of the rows in the outer joined tables by the total number of rows in the group:

select fp.age, 
       (count(cl.id)::numeric / count(*)::numeric) * 100 as ration,
       count(cl.id) as class_count, 
       count(*) as age_count
from football_players fp
  left join class cl on fp.id = cl.id
group by fp.age

The cast ::numeric is necessary to get fractional values, otherwise the result of the division will be converted to an integer.

SQLFiddle: http://sqlfiddle.com/#!15/3373c/3