PostgreSQL: Find percentages of total_films_rented - postgresql

Question

The code below gives me the following results
Early: 7738
Late: 6586
On Time: 1720
How would I take this a step further and add a third column that finds the percentages?
Here is a link to the ERD and database set-up: https://www.postgresqltutorial.com/postgresql-sample-database/
WITH
t1
AS
(
SELECT *, DATE_PART('day', return_date - rental_date) AS days_rented
FROM rental
),
t2
AS
(
SELECT rental_duration, days_rented,
CASE WHEN rental_duration > days_rented THEN 'Early'
WHEN rental_duration = days_rented THEN 'On Time'
ELSE 'Late'
END AS rental_return_status
FROM film f, inventory i, t1
WHERE f.film_id = i.film_id AND t1.inventory_id = i.inventory_id
)
SELECT rental_return_status, COUNT(*) AS total_films_rented
FROM t2
GROUP BY 1
ORDER BY 2 DESC;

Answer 1

You can use a window function with one CTE table (instead of 2):
WITH raw_status AS (
SELECT rental_duration - DATE_PART('day', return_date - rental_date) AS days_remaining
FROM rental r
JOIN inventory i ON r.inventory_id=i.inventory_id
JOIN film f on f.film_id=i.film_id
)
SELECT CASE WHEN days_remaining > 0 THEN 'Early'
WHEN days_remaining = 0 THEN 'On Time'
ELSE 'Late' END AS rental_status,
count(*),
(100*count(*))/sum(count(*)) OVER () AS percentage
FROM raw_status
GROUP BY 1;
rental_status | count | percentage
---------------+-------+---------------------
Early | 7738 | 48.2298678633757168
On Time | 1720 | 10.7205185739217153
Late | 6586 | 41.0496135627025679
(3 rows)
Disclosure: I work for EnterpriseDB (EDB)

Answer 2

Use a window function to get the sum of the count column (sum(count(*)) over ()), then just divide the count by that (count(*)/sum(count(*)) over ()). Multiply by 100 to make it a percentage.
psql (12.1 (Debian 12.1-1))
Type "help" for help.
testdb=# CREATE TABLE faket2 AS (
SELECT 'early' AS rental_return_status UNION ALL
SELECT 'early' UNION ALL
SELECT 'ontime' UNION ALL
SELECT 'late');
SELECT 4
testdb=# SELECT
rental_return_status,
COUNT(*) as total_films_rented,
(100*count(*))/sum(count(*)) over () AS percentage
FROM faket2
GROUP BY 1
ORDER BY 2 DESC;
rental_return_status | total_films_rented | percentage
----------------------+--------------------+---------------------
early | 2 | 50.0000000000000000
late | 1 | 25.0000000000000000
ontime | 1 | 25.0000000000000000
(3 rows)