You want to find the average order amount per customer but only for customers who placed more than 3 orders. Which query correctly achieves this?

hard📝 Application Q15 of 15

PostgreSQL - Aggregate Functions and GROUP BY

ASELECT customer_id, AVG(amount) FROM orders WHERE COUNT(*) > 3 GROUP BY customer_id;

BSELECT customer_id, AVG(amount) FROM orders HAVING COUNT(*) > 3 GROUP BY customer_id;

CSELECT customer_id, AVG(amount) FROM orders GROUP BY customer_id WHERE COUNT(*) > 3;

DSELECT customer_id, AVG(amount) FROM orders GROUP BY customer_id HAVING COUNT(*) > 3;

Step-by-Step Solution

Solution:

Step 1: Understand filtering groups with HAVING
HAVING filters groups after aggregation; WHERE filters rows before aggregation.
Step 2: Check query order and clauses
SELECT customer_id, AVG(amount) FROM orders GROUP BY customer_id HAVING COUNT(*) > 3; correctly uses GROUP BY then HAVING COUNT(*) > 3 to filter customers with more than 3 orders.
Final Answer:
SELECT customer_id, AVG(amount) FROM orders GROUP BY customer_id HAVING COUNT(*) > 3; -> Option D
Quick Check:
Use HAVING to filter groups after GROUP BY [OK]

Quick Trick: Use HAVING to filter groups, not WHERE [OK]

Common Mistakes:

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