- Write a query to return the number of
in demandandnot in demandmovies in May 2020. - Assumptions (great to clarify in your interview): all films are available for rent before May.
- But if a film is not in stock, it is not in demand.
- The order of your results doesn't matter.
in-demand: rented>1 timesin May 2020.not-in-demand: rented<= 1 timein May 2020.
Table 1: film
col_name | col_type
----------------------+--------------------------
film_id | integer
title | text
description | text
release_year | integer
language_id | smallint
original_language_id | smallint
rental_duration | smallint
rental_rate | numeric
length | smallint
replacement_cost | numeric
rating | text
Table 2: inventory
Each row is unique, inventoy_id is the primary key of this table.
col_name | col_type --------------+-------------------------- inventory_id | integer film_id | smallint store_id | smallint
Table 3: rental
col_name | col_type --------------+-------------------------- rental_id | integer rental_ts | timestamp with time zone inventory_id | integer customer_id | smallint return_ts | timestamp with time zone staff_id | smallint
Sample results
demand_category | count -----------------+------- in-demand | 123 not-in-demand | 456
Expected results
Solution postgres
SELECT demand_category, COUNT(*)
FROM (
SELECT
F.film_id,
CASE WHEN COUNT(R.rental_id) >1 THEN 'in demand' ELSE 'not in demand' END AS demand_category
FROM film F
LEFT JOIN INVENTORY I
ON F.film_id =I.film_id
LEFT JOIN (
SELECT inventory_id, rental_id
FROM rental
WHERE DATE(rental_ts) >= '2020-05-01'
AND DATE(rental_ts) <= '2020-05-31'
) R
ON R.inventory_id = I.inventory_id
GROUP BY F.film_id
)X
GROUP BY demand_category;
Explanation
This query is used to count the number of films that were either "in demand" or "not in demand" during May 2020.
The first part of the query is a subquery that joins the film table with the inventory table and the rental table (filtered for rentals between May 1 and May 31). It groups by film_id and assigns a category of "in demand" if the film was rented more than once during that time period, and "not in demand" if it wasn't rented at all or only rented once.
The outer query then takes the results of the subquery and groups them by the demand category, and counts the number of films in each category.
Overall, this query is useful for analyzing film demand during a specific time period and identifying trends in rental patterns.