Calculating percentage over sub query SQL

Question

I have a table that records events regarding email campaigns. I want to figure out the percentage of campaigns where there was more than one event happening for the campaign.

First I calculated the number of events happening in each campaign:

select count(*) as counter
               from campaigns_log
               where event IN ('send', 'open')
                 and campaign_id is not null
               group by campaign_id, email

Then I grouped the campaigns in the condition whether more than one campaign happened:

select count(counter) as occurences, IF(counter > 1, 2, 1) as grouper
         from (select count(*) as counter
               from campaigns_log
               where event IN ('send', 'open')
                 and campaign_id is not null
               group by campaign_id, email) as counters_table
         group by grouper

Sample result:

occurences ¦ grouper
132        ¦ 1
360        ¦ 2

Now I want to calculate for each row the percentage of total occurrences. So something like this:

occurences ¦ grouper ¦ percentage
132        ¦ 1       ¦ 132/(132+360)
360        ¦ 2       ¦ 360/(132+360)

I tried this, but it does not work, it does not properly calculate the sum total:

select *, occurences/(select sum(occurences))
from (
         select count(counter) as occurences, IF(counter > 1, 2, 1) as grouper
         from (select count(*) as counter
               from campaigns_log
               where event IN ('send', 'open')
                 and campaign_id is not null
               group by campaign_id, email) as counters_table
         group by grouper
     ) as occurences_table group by occurences, grouper

Any idea where is my mistake in the last step?

Answer 1

Part of the secret is

SELECT  (...)/total, ...
    FROM campaigns_log
    JOIN ( SELECT SUM(...) AS total FROM ... ) x

That is, compute the total separately and make it available in the expressions.

Similarly:

SELECT @total := SUM(...) FROM ...
SELECT (...)/@total, ...