Calculate Percentage with JOINS and original value I need your help. I'm trying to calculate percentages by using results from JOINING divided by the original value. I do not know how to combine both values. I try to find trips with station id which do not exist in the table station and then find the percentage of that trips.
I only can write the query to this extend:
SELECT COUNT(*)
FROM (SELECT a.start_station_id, b.station_id, a.end_station_id,a.bikeid
FROM `bigquery-public-data.new_york_citibike.citibike_trips` as a
LEFT OUTER JOIN  `bigquery-public-data.new_york_citibike.citibike_stations` as b 
ON a.start_station_id = b.station_id
WHERE b.station_id IS NULL)

I need to divide the result COUNT from the query above with COUNT(*) before JOIN.
Kindly help me with this.
Thank you!!
One way to get expected output can be
select case when total_count = 0 then 0.0 else count_1*100.0/total_count end as pcnt 
from 
(
SELECT count(case when b.statition_id is null then 1 end) as count_1, 
count(*) as total_count 
FROM `bigquery-public-data.new_york_citibike.citibike_trips` as a
LEFT OUTER JOIN  `bigquery-public-data.new_york_citibike.citibike_stations` as b 
ON a.start_station_id = b.station_id
)x 

Note, it returns 0.0 if total_count is 0 to avoid division by zero.  You may want to return null(casted to proper decimal type ) in such case instead.