1045. Customers Who Bought All Products

Level: Medium; Amazon

Question:

Table: Customer

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| customer_id | int     |
| product_key | int     |
+-------------+---------+
There is no primary key for this table. It may contain duplicates.
product_key is a foreign key to Product table.

Table: Product

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| product_key | int     |
+-------------+---------+
product_key is the primary key column for this table.

Write an SQL query to report the customer ids from the Customer table that bought all the products in the Product table.

Return the result table in any order.

The query result format is in the following example.

Example 1:

Input: 
Customer table:
+-------------+-------------+
| customer_id | product_key |
+-------------+-------------+
| 1           | 5           |
| 2           | 6           |
| 3           | 5           |
| 3           | 6           |
| 1           | 6           |
+-------------+-------------+
Product table:
+-------------+
| product_key |
+-------------+
| 5           |
| 6           |
+-------------+
Output:
 
+-------------+
| customer_id |
+-------------+
| 1           |
| 3           |
+-------------+
Explanation:
 
The customers who bought all the products (5 and 6) are customers with IDs 1 and 3.

My Solution:

Use a subquery to count the total number of product; Use another subquery to count the distinct products each customer bought; Compare the number.

select customer_id
from (select count(distinct product_key) as number from Product) p, 
(select customer_id, count(distinct product_key) as number
from Customer 
group by customer_id) t
where p.number=t.number