Writing Data Tests

SELECT order_id, buyer_id, seller_id, status
FROM orders
WHERE buyer_id IS NULL
ORDER BY order_id;

Output:

+----------+----------+-----------+--------+
| order_id | buyer_id | seller_id | status |
+----------+----------+-----------+--------+
|     1003 | NULL     | S-1       | paid   |
+----------+----------+-----------+--------+

This query returned one row, and that row is the problem it was written to find. Order 1003 has no buyer, so no customer can be charged or sent a receipt for it. An empty result would have meant that the table passed the check.

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