solve: questions faizanxmulla#13, #14

Author: faizanxmulla

ankit-bansal-sql/complex-sql-questions-playlist/13-recommendation-system.md

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+### Problem Statement
+
+Write an SQL query to find all pairs of products that are purchased together by the same customer in the same order. 
+
+For each pair, return the product names and the frequency with which the pair was purchased together.
+
+### Schema Setup
+
+```sql
+CREATE TABLE orders (
+    order_id INT,
+    customer_id INT,
+    product_id INT
+);
+
+INSERT INTO orders (order_id, customer_id, product_id) VALUES
+(1, 1, 1),
+(2, 1, 2),
+(3, 1, 3),
+(4, 2, 1),
+(5, 2, 2);
+
+
+CREATE TABLE products (
+    id INT PRIMARY KEY,
+    name VARCHAR(50)
+);
+
+INSERT INTO products (id, name) VALUES
+(1, 'A'),
+(2, 'B'),
+(3, 'C'),
+(4, 'D'),
+(5, 'E');
+```
+
+### Expected Output
+
+| pair | purchase_freq |
+|------|---------------|
+| A B  | 2             |
+| A C  | 1             |
+| B C  | 1             |
+| A D  | 1             |
+| B D  | 1             |
+
+
+
+### Solution Query 
+
+```sql
+SELECT   CONCAT(pr1.name, ' ' , pr2.name) AS pair, COUNT(*) AS purchase_freq 
+FROM     orders o1 JOIN orders o2 ON o1.order_id = o2.order_id 
+                   JOIN products pr1 ON pr1.id = o1.product_id 
+                   JOIN products pr2 ON pr2.id = o2.product_id
+WHERE    o1.product_id < o2.product_id
+GROUP BY 1
+```

ankit-bansal-sql/complex-sql-questions-playlist/14-prime-subscription.md

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+### Problem Statement
+
+Given the `users` and `events` tables, return the fraction of users, rounded to two decimal places, who accessed Amazon Music and upgraded to Prime membership within the first 30 days of signing up.
+
+The output should return a single value representing the fraction of users who meet the above criteria.
+
+
+### Schema Setup
+
+```sql
+CREATE TABLE users (
+    user_id INT PRIMARY KEY,
+    name VARCHAR(50),
+    join_date DATE
+);
+
+INSERT INTO users (user_id, name, join_date) VALUES
+(1, 'Jon', '2020-02-14'),
+(2, 'Jane', '2020-02-14'),
+(3, 'Jill', '2020-02-14'),
+(4, 'Josh', '2020-02-15'),
+(5, 'Jean', '2020-02-16'),
+(6, 'Justin', '2020-02-17'),
+(7, 'Jeremy', '2020-02-18');
+
+CREATE TABLE events (
+    user_id INT,
+    type VARCHAR(50),
+    access_date DATE
+);
+
+INSERT INTO events (user_id, type, access_date) VALUES
+(1, 'Pay', '2020-03-01'),
+(2, 'MLack', '2020-03-02'),
+(2, 'P', '2020-03-12'),
+(3, 'Music', '2020-03-15'),
+(3, 'P', '2020-03-15'),
+(6, 'P', '2020-03-16'),
+(7, 'P', '2020-03-22');
+```
+
+
+### Expected Output
+
+fraction |
+--|
+33.33 |
+
+### Solution Query
+
+```sql
+WITH events_cte AS (
+    SELECT   user_id, 
+             access_date,
+             COUNT(*) FILTER(WHERE type='Music') as music_members, 
+             COUNT(*) FILTER(WHERE type='P') as prime_members
+    FROM     events
+    GROUP BY 1, 2
+)
+SELECT 100.0 * ROUND(SUM(music_members) / SUM(prime_members), 2) as fraction
+FROM   events_cte e JOIN users u USING(user_id)
+WHERE  access_date <= join_date + INTERVAL '30 day'
+```