LeetCode 1045: Customers Who Bought All Products

Problem Restatement

We have two tables:

Customer(customer_id, product_key) — records each purchase.

Product(product_key) — all products in the catalog.

We need to return the customer_id of customers who bought all products in the catalog.

Input and Output

Return a table with one column: customer_id.

Examples

Customer:               Product:
+-------------+----------+      +-------------+
| customer_id | product_key |    | product_key |
+-------------+----------+      +-------------+
| 1           | 5         |    | 5           |
| 2           | 6         |    | 6           |
| 3           | 5         |    +-------------+
| 3           | 6         |
| 1           | 6         |
+-------------+----------+

Customers who bought both 5 and 6: customers 1 and 3.

Answer:

+-------------+
| customer_id |
+-------------+
| 1           |
| 3           |
+-------------+

Key Insight

For a customer to have bought all products, the count of distinct products they purchased must equal the total number of products.

Group by customer_id and filter with HAVING COUNT(DISTINCT product_key) = (SELECT COUNT(*) FROM Product).

Edge Cases

• Match the aggregate to the question: use raw COUNT or SUM for rows/amounts, and COUNT(DISTINCT ...) only for unique entities.
• Tie cases should return every qualifying row unless the statement asks for a single row.
• For outer joins, keep filters on the joined table in the ON clause when unmatched rows must remain visible.

Common Pitfalls

• Do not put aggregate filters in WHERE; use HAVING after GROUP BY.
• When the answer needs all tied winners, avoid LIMIT 1 unless the statement explicitly asks for one row.

Implementation

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

Code Explanation

GROUP BY customer_id aggregates each customer’s purchases.

COUNT(DISTINCT product_key) counts unique products each customer bought.

(SELECT COUNT(*) FROM Product) gives the total number of products.

The HAVING clause keeps only customers who bought every product.