MarketAnalysisII.sql

/* 1159. Market Analysis II
SQL Schema
Table: Users

+----------------+---------+
| Column Name    | Type    |
+----------------+---------+
| user_id        | int     |
| join_date      | date    |
| favorite_brand | varchar |
+----------------+---------+
user_id is the primary key of this table.
This table has the info of the users of an online shopping website where users can sell and buy items.
 

Table: Orders

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| order_id      | int     |
| order_date    | date    |
| item_id       | int     |
| buyer_id      | int     |
| seller_id     | int     |
+---------------+---------+
order_id is the primary key of this table.
item_id is a foreign key to the Items table.
buyer_id and seller_id are foreign keys to the Users table.
 

Table: Items

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| item_id       | int     |
| item_brand    | varchar |
+---------------+---------+
item_id is the primary key of this table.
 

Write an SQL query to find for each user whether the brand of the second item (by date) they sold is their favorite brand. If a user sold less than two items, report the answer for that user as no. It is guaranteed that no seller sold more than one item on a day.

Return the result table in any order.

The query result format is in the following example.

 

Example 1:

Input: 
Users table:
+---------+------------+----------------+
| user_id | join_date  | favorite_brand |
+---------+------------+----------------+
| 1       | 2019-01-01 | Lenovo         |
| 2       | 2019-02-09 | Samsung        |
| 3       | 2019-01-19 | LG             |
| 4       | 2019-05-21 | HP             |
+---------+------------+----------------+
Orders table:
+----------+------------+---------+----------+-----------+
| order_id | order_date | item_id | buyer_id | seller_id |
+----------+------------+---------+----------+-----------+
| 1        | 2019-08-01 | 4       | 1        | 2         |
| 2        | 2019-08-02 | 2       | 1        | 3         |
| 3        | 2019-08-03 | 3       | 2        | 3         |
| 4        | 2019-08-04 | 1       | 4        | 2         |
| 5        | 2019-08-04 | 1       | 3        | 4         |
| 6        | 2019-08-05 | 2       | 2        | 4         |
+----------+------------+---------+----------+-----------+
Items table:
+---------+------------+
| item_id | item_brand |
+---------+------------+
| 1       | Samsung    |
| 2       | Lenovo     |
| 3       | LG         |
| 4       | HP         |
+---------+------------+
Output: 
+-----------+--------------------+
| seller_id | 2nd_item_fav_brand |
+-----------+--------------------+
| 1         | no                 |
| 2         | yes                |
| 3         | yes                |
| 4         | no                 |
+-----------+--------------------+
Explanation: 
The answer for the user with id 1 is no because they sold nothing.
The answer for the users with id 2 and 3 is yes because the brands of their second sold items are their favorite brands.
The answer for the user with id 4 is no because the brand of their second sold item is not their favorite brand.

 */
 
--get 2nd order records

WITH temp1 AS( 
SELECT
    u.user_id, 
    CASE WHEN u.favorite_brand=i.item_brand THEN 'yes' ELSE 'no'
            END AS 2nd_order,
    ROW_NUMBER() OVER(PARTITION BY u.user_id ORDER BY o.order_date) rn    
    FROM Users u
        join Orders o
            ON u.user_id = o.seller_id
        join Items i
            ON o.item_id = i.item_id)

-- left join with Users table

SELECT 
    u1.user_id as seller_id,
    COALESCE(u2.2nd_order,'no') AS 2nd_item_fav_brand
    FROM
        Users u1 
        LEFT JOIN
        (SELECT 
            user_id,2nd_order
                from temp1
                WHERE rn=2
         )u2
        ON u1.user_id = u2.user_id
        ;