1045. Customers Who Bought All Products
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
Thinking
We need customers who bought every product in the catalog. After grouping by customer, the number of distinct products should equal the size of the product table.
Group Customer by customer_id and compare COUNT(DISTINCT product_key) to a subquery that counts Product.
The subquery evaluates the global total once, instead of pairing the catalog with each customer row.
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.
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