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SQL for Beginners: Ask Your Data Straight Questions
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SQL for Beginners: Ask Your Data Straight Questions

Check Yourself: Combining Tables

Five scenario questions on keys, foreign keys, LEFT JOIN and fan-out. If you miss more than one, reread the lessons in this section.

5 min SQLDatabases Beginner
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1. Why does the order_items table have a primary key made of two columns, order_id and product_id?
Because order_id alone could not be a number. Because a primary key must always have exactly two columns. Because two columns are faster than one. Each product can appear at most once in an order, so the pair identifies a row.
2. INSERT INTO orders with customer_id 99 is refused. Which feature stopped it?
The ORDER BY clause. The primary key on orders. The foreign key from orders.customer_id to customers.customer_id. The default value of the status column.
3. You need the names of customers who have no orders. Which query shape gives them?
A CROSS JOIN between customers and orders. A LEFT JOIN from customers to orders, keeping rows where the order id is NULL. An INNER JOIN from customers to orders. A GROUP BY on the orders table alone.
4. You write customers LEFT JOIN orders and then WHERE o.status = 'paid'. Customers with no paid orders disappear. Why?
LEFT JOIN only works when the right table has no WHERE. PostgreSQL always changes a LEFT JOIN into an INNER JOIN. The unmatched rows have a NULL status, the comparison is unknown, and WHERE drops them. The word paid is a reserved word.
5. There are 12 orders. After joining orders to order_items, count(*) returns 20. Is the data wrong?
Yes. The order_items table contains 8 duplicate rows. No. count(*) always returns the larger of the two tables. No. Each order is repeated once per item, so count(DISTINCT order_id) gives 12. Yes. An INNER JOIN always adds 8 rows.

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Five scenario questions on keys, foreign keys, LEFT JOIN and fan-out. If you miss more than one, reread the lessons in this section.

Duration
5 min
Last update
09/24/2026
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