1045. Customers Who Bought All Products

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:

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.

解題思路：
把product表未重複的數量跟customer_id表未重複的數量相同當條件
去搜會比較好想


SELECT customer_id 
FROM Customer
GROUP BY customer_id
HAVING ( COUNT(DISTINCT product_key) ) = ( SELECT COUNT(DISTINCT product_key )FROM Product )

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