diff --git a/functions.sql b/functions.sql
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+++ b/functions.sql
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+USE sakila;
+
+-- Challenge 1
+
+-- 1. Rank films by length:
+SELECT title, length, RANK() OVER (ORDER BY length DESC) AS ranki
+FROM film
+WHERE length IS NOT NULL AND length > 0;
+
+-- 2. Rank films by length within rating category:
+SELECT title, length, rating, RANK() OVER (PARTITION BY rating ORDER BY length DESC) AS ranki
+FROM film
+WHERE length IS NOT NULL AND length > 0;
+
+-- 3. List actors/actresses with the most films:
+WITH ActorFilmCounts AS (
+    SELECT actor_id, COUNT(*) AS film_count
+    FROM film_actor
+    GROUP BY actor_id
+),
+ActorInfo AS (
+    SELECT a.actor_id, a.first_name, a.last_name, afc.film_count
+    FROM actor a
+    JOIN ActorFilmCounts afc ON a.actor_id = afc.actor_id
+)
+SELECT *
+FROM ActorInfo
+ORDER BY film_count DESC;
+
+-- Challenge 2
+
+-- Step 1. Retrieve the number of monthly active customers:
+SELECT MONTH(rental_date) AS month, COUNT(DISTINCT customer_id) AS active_customers
+FROM rental
+GROUP BY MONTH(rental_date);
+
+-- Step 2. Retrieve the number of active users in the previous month:
+WITH MonthlyActiveCustomers AS (
+    SELECT MONTH(rental_date) AS month, COUNT(DISTINCT customer_id) AS active_customers
+    FROM rental
+    GROUP BY MONTH(rental_date)
+)
+SELECT
+    mac.month,
+    mac.active_customers,
+    LAG(mac.active_customers) OVER (ORDER BY mac.month) AS prev_month_active_customers
+FROM
+    MonthlyActiveCustomers mac;
+    
+-- Step 3. Calculate the percentage change in the number of active customers:
+WITH MonthlyActiveCustomers AS (
+    SELECT MONTH(rental_date) AS month, COUNT(DISTINCT customer_id) AS active_customers
+    FROM rental
+    GROUP BY MONTH(rental_date)
+),
+ActiveCustomersWithPrevious AS (
+    SELECT
+        mac.month,
+        mac.active_customers,
+        LAG(mac.active_customers) OVER (ORDER BY mac.month) AS prev_month_active_customers
+    FROM
+        MonthlyActiveCustomers mac
+)
+SELECT
+    month,
+    active_customers,
+    prev_month_active_customers,
+    ((active_customers - prev_month_active_customers) / prev_month_active_customers) * 100 AS pct_change
+FROM
+    ActiveCustomersWithPrevious;
+    
+-- Step 4. Calculate the number of retained customers:
+WITH MonthlyActiveCustomers AS (
+    SELECT 
+        MONTH(rental_date) AS month, 
+        COUNT(DISTINCT customer_id) AS active_customers
+    FROM 
+        rental
+    GROUP BY 
+        MONTH(rental_date)
+),
+ActiveCustomersWithPrevious AS (
+    SELECT
+        mac.month,
+        mac.active_customers,
+        LAG(mac.active_customers) OVER (ORDER BY mac.month) AS prev_month_active_customers
+    FROM
+        MonthlyActiveCustomers mac
+),
+MonthlyCustomerRetention AS (
+    SELECT
+        MONTH(rental_date) AS month,
+        customer_id,
+        LAG(MONTH(rental_date)) OVER (PARTITION BY customer_id ORDER BY rental_date) AS prev_month
+    FROM 
+        rental
+),
+RetainedCustomers AS (
+    SELECT 
+        month,
+        COUNT(DISTINCT customer_id) AS retained_customers
+    FROM 
+        MonthlyCustomerRetention
+    WHERE 
+        prev_month IS NOT NULL
+    GROUP BY 
+        month
+)
+SELECT
+    acwp.month,
+    acwp.active_customers,
+    acwp.prev_month_active_customers,
+    rc.retained_customers
+FROM
+    ActiveCustomersWithPrevious acwp
+LEFT JOIN
+    RetainedCustomers rc ON acwp.month = rc.month
+ORDER BY
+    acwp.month;
+