How can I calculate the AVG of the below CTE? - sql

Question

I am trying to find the average of a query which has 600,000 rows. I have created a CTE to calculate distances based off of longitude and latitude. Is it possible for me to add an aggregate function to this, as the calculated_distance is not an existing column?
Thanks in advance!
WITH name AS (
SELECT
id,
latitude,
longitude,
name,
docks
FROM
santander_stations
),
ride_data AS (
SELECT
startstationid,
endstationid
FROM
public.santander_2016
UNION
SELECT
startstationid,
endstationid
FROM
public.santander_2017
UNION
SELECT
startstationid,
endstationid
FROM
public.santander_2018
)
SELECT
calculate_distance( a.latitude, a.longitude, b.latitude, b.longitude, 'K' ),
a.name AS Start_Station,
b.name AS End_Station
FROM
name AS a,
name AS b
WHERE
a.id IN ( SELECT startstationid FROM ride_data )
AND
b.id IN ( SELECT endstationid FROM ride_data )
ORDER BY
1 DESC

Answer 1

The number of rides is much more than the number of possible start & end stations. Therefore, we can calculate the distances of all combinations of (start station, end station) first, and then use the result as a lookup table for the 600,000 rides.
Here's a way to calculate the average ride distance in year 2016, 2017 and 2018 (please replace (1). the distance_km formula with your calculated_distance() UDF, (2). Santander_rides with your ride_data subquery)
with cte_santander_station_distance as (
select x.id as start_station_id,
y.id as end_station_id,
acos(sin(x.latitude)*sin(y.latitude)+cos(x.latitude)*cos(y.latitude)*cos(y.longitude-x.longitude)) as distance_km
from santander_stations x, santander_stations y
where x.id <> y.id)
select avg(ssd.distance_km) as average_distance_km,
count(*) as rides
from santander_rides sr
join cte_santander_station_distance ssd
on sr.start_station_id = ssd.start_station_id
and sr.end_station_id = ssd.end_station_id;
If there're lots of combinations of start station and end station, you can materialize the lookup table such as below: