mathematical operation between two select statements

I’m currently trying to find the percentage of certain amount of preregistered users in my postgres db, the operation would be (1185 * 100) / 3104 = 38.17. To do that I’m using two select statements to retrieve each count, but I’ve been unable to operate between them:

+ count +
-  1185 -
-  3104 -

This is what I have:

select
    count(*)
    from crm_user_preregistred
    left join crm_player on crm_user_preregistred."document" = crm_player."document" 
    left join crm_user on crm_player.user_id = crm_user.id
    where crm_user.email is not null
union all
select
    count(*)
    from crm_user_preregistred

Thanks in advance for any hint or help.

Answer

you can use some with clause to simplifie your selects, substitute the values with your count(*) selects, maybe some formating to the result, and a check for 0 on value2

with temp_value1 as (
   select 1185 as value1 ),
temp_value2 as (
select 3104 as value2 )

select (select temp_value1.value1::float * 100  from temp_value1) /
(select temp_value2.value2::float from temp_value2)

result : 38.17654639175258

with your selects:

with temp_value1 as (
select
    count(*) as value1
    from crm_user_preregistred
    left join crm_player on crm_user_preregistred."document" = crm_player."document" 
    left join crm_user on crm_player.user_id = crm_user.id
    where crm_user.email is not null
),
temp_value2 as (
select
    count(*) as value2
    from crm_user_preregistred
)

select (select temp_value1.value1::float * 100  from temp_value1)  / (select temp_value2.value2::float from temp_value2)