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CODE WORKSPACE
Return c.customer_id and c.country for customers who have at least one shipped order. Each customer should appear once. Order by customer_id.
Result columns · in this order
customer_id | Stable customer identifier. |
country | Customer country. Can be NULL when unknown. |
How to approach it
WHERE EXISTS (SELECT 1 FROM orders o WHERE o.buyer_id = c.customer_id AND o.status = 'shipped').
Sample input
| customer_id | country |
|---|---|
| 1 | US |
| 2 | null |
| 3 | GB |
| 4 | IN |
| 5 | US |
| 6 | GB |
6 rows — all rows shown.
| order_id | buyer_id | status |
|---|---|---|
| 1004 | 1 | shipped |
| 1008 | 4 | shipped |
| 1012 | 3 | shipped |
3 rows — all rows shown.
Expected output
| customer_id | country |
|---|---|
| 1 | US |
| 3 | GB |
| 4 | IN |
3 rows — all rows shown.
Constraints
Use EXISTS with a correlated subquery on orders (a semi-join) so a customer with several shipped orders still appears once. Do not use a JOIN that would multiply the customer rows.
Expected skills
Semi-join with EXISTS for existence questions, returning each left row at most once and avoiding fan-out.
Submit for review to find out what your query gets right, what it gets wrong, and how it compares with the best working query for this exercise.
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