SQL practice: How a Database Thinks

Which column in orders is its primary key? Which columns are foreign keys?

order_items has a column called unit_price, and so does products. Why store the price twice instead of always looking it up in products?

If customer_id 3 is deleted from customers, what should happen to their orders? What does the foreign key do about it?

Explain in one sentence why "every query returns a table" makes subqueries possible.

Read this aloud in plain English: SELECT city FROM customers WHERE customer_type = 'Dealer';

employees.manager_id points at employees.employee_id — the same table. What real-world fact is that recording?