What are UNION, INTERSECT, and EXCEPT?

-- All customers and suppliers
SELECT name, city FROM customers
UNION
SELECT name, city FROM suppliers;

-- UNION ALL keeps duplicates (faster)
SELECT name FROM customers
UNION ALL
SELECT name FROM suppliers;

-- Cities with both customers and suppliers
SELECT city FROM customers
INTERSECT
SELECT city FROM suppliers;

-- Customers not in suppliers
SELECT name FROM customers
EXCEPT
SELECT name FROM suppliers;

-- Products never ordered
SELECT id FROM products
EXCEPT
SELECT DISTINCT product_id FROM order_items;

-- Combine multiple status queries
SELECT id, name, 'VIP' as customer_type
FROM customers WHERE total_purchases > 10000

UNION ALL

SELECT id, name, 'Regular'
FROM customers WHERE total_purchases BETWEEN 1000 AND 10000

UNION ALL

SELECT id, name, 'New'
FROM customers WHERE total_purchases < 1000

ORDER BY customer_type, name;