WITH CTE query using postgresql - sql

Question

I am learning how to use SQL CTE and I would like to compare two query to have the same answer (using postgresql) but I fail can someone help plese?
I create this query and I have the total of each film title (Sakila database):
SELECT COUNT(r.rental_id) rental_count,
f.title as "Film"
FROM film f
JOIN inventory i
ON f.film_id = i.film_id
JOIN rental r USING (inventory_id)
GROUP BY f.title
ORDER BY rental_count DESC;
I would like to do the same using the WITH (CTE) and for that I create this code :
WITH table1 AS (
SELECT f.film_id,
f.title as "Film"
FROM film f),
table2 AS (
SELECT r.inventory_id,
COUNT(r.rental_id) rental_count,
i.film_id,
i.inventory_id
FROM inventory i
JOIN rental r USING (inventory_id)
GROUP BY r.inventory_id, i.film_id, i.inventory_id)
SELECT *
FROM table1
JOIN table2
ON table1.film_id = table2.film_id;
The problem is that the result did not show the total of each film title, but instead every film title separately.

Answer 1

The second CTE would need to be grouped by film to produce an equivalent end result.
WITH table1 AS (
SELECT
f.film_id
, f.title AS "Film"
FROM film f
)
, table2 AS (
SELECT
COUNT(r.rental_id) rental_count
, i.film_id
FROM inventory i
JOIN rental r ON i.inventory_id = r.inventory
GROUP BY i.film_id
)
SELECT
table2.rental_count
, table1.Film
FROM table1
JOIN table2 ON table1.film_id = table2.film_id
ORDER BY rental_count DESC;
Just a note; I would not recommend using both natural and non-natural join types in a single query, it can get quite confusing.
SELECT
COUNT(r.rental_id) rental_count
, f.title AS "Film"
FROM film f
JOIN inventory i ON f.film_id = i.film_id
JOIN rental r ON i.inventory_id = r.inventory_id -- change here
GROUP BY f.title
ORDER BY rental_count DESC;

Answer 2

To get the same result, you'd have to aggregate and group in the second query just like in the first:
WITH table1 AS (...),
table2 AS (...)
SELECT count(table2.rental_count) AS rental_count,
table1."Film"
FROM table1
JOIN table2 USING (film_id)
GROUP BY table1."Film"
ORDER BY rental_count DESC;
Basically you use the CTEs instead of the original tables.