You want to find duplicate entries in a table orders with columns order_id, product_id, and customer_id. Which query efficiently finds product_id values ordered by more than one customer?

hard📝 Application Q8 of 15

SQL - Advanced Query Patterns

ASELECT product_id FROM orders GROUP BY product_id HAVING COUNT(DISTINCT customer_id) > 1;

BSELECT product_id FROM orders WHERE COUNT(customer_id) > 1;

CSELECT product_id FROM orders GROUP BY product_id HAVING COUNT(customer_id) > 1;

DSELECT product_id FROM orders GROUP BY customer_id HAVING COUNT(product_id) > 1;

Step-by-Step Solution

Solution:

Step 1: Understand the requirement
We want product_ids ordered by more than one distinct customer.
Step 2: Use COUNT(DISTINCT customer_id) with GROUP BY product_id
This counts unique customers per product; HAVING > 1 filters duplicates.
Final Answer:
SELECT product_id FROM orders GROUP BY product_id HAVING COUNT(DISTINCT customer_id) > 1; -> Option A
Quick Check:
COUNT(DISTINCT) finds unique customers per product [OK]

Quick Trick: Use COUNT(DISTINCT column) to count unique values in groups [OK]

Common Mistakes:

Using WHERE with COUNT
Counting without DISTINCT
Grouping by wrong column

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You want to find duplicate entries in a table orders with columns order_id, product_id, and customer_id. Which query efficiently finds product_id values ordered by more than one customer?

Step 1: Understand the requirement

Step 2: Use COUNT(DISTINCT customer_id) with GROUP BY product_id

Final Answer:

Quick Check:

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