Buyers Without Repeats
CRM needs a list of users who have at least one order. Do not repeat the same buyer, and guest orders without user_id should produce one separate row. Return user_id in ascending order, placing the empty value last.
Expected output sample
This is what a correct answer looks like — its row count is its own, it doesn't have to match the schema tables.
| user_id |
|---|
| 1 |
| 2 |
| 3 |
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