Question

1045. Customers Who Bought All Products

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Description

Table: Customer

+-------------+---------+
| 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

+-------------+---------+
| Column Name | Type  |
+-------------+---------+
| product_key | int   |
+-------------+---------+
product_key is the primary key (column with unique values) for this table.

 

Write a solution 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 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.

Solutions

Solution 1: Grouping and Subquery

We can group the Customer table by customer_id, and then use the HAVING clause to filter out the customers who have not purchased all products. To do this, we can use a subquery to find the total number of distinct products, and then compare it with the number of distinct products purchased by each customer.

# Write your MySQL query statement below
SELECT customer_id
FROM Customer
GROUP BY 1
HAVING COUNT(DISTINCT product_key) = (SELECT COUNT(1) FROM Product);