Find Observed Customers Who Purchased the Entire Product Set
Company: Meta
Role: Software Engineer
Category: Data Manipulation (SQL/Python)
Difficulty: medium
Interview Round: Other
### Problem
Return customer_id values appearing in Customer that bought every product_key in Product. Repeated Customer rows do not represent additional distinct products. Return each qualifying customer once, in any order. A NULL product key records no purchase. If Product is empty, every observed customer qualifies.
### Tables
Table: `Customer`
```text
+-------------+---------+
| Column Name | Type |
+-------------+---------+
| customer_id | int |
| product_key | int |
+-------------+---------+
This table may contain duplicates rows.
customer_id is not NULL.
product_key is a foreign key (reference column) to Product table.
```
Table: `Product`
```text
+-------------+---------+
| Column Name | Type |
+-------------+---------+
| product_key | int |
+-------------+---------+
product_key is the primary key (column with unique values) for this table.
```
### Example
```text
Product (product_key): (8), (9)
Customer (customer_id,product_key): (1,8), (1,8), (1,9), (2,8), (3,9)
Output (customer_id): (1)
```
### What a Strong Answer Covers
- Compare each observed customer's distinct purchases with the full product set.
- Return each qualifying customer once and handle repeated purchases and an empty product list.
### Follow-up Questions
- How would you report the products each customer is missing?
- How would you require only products in a particular category?
Overview: Find customers who purchased every product, counting repeated purchases of the same product only once.