1. SELECT * FROM order_table WHERE customer_id IS NULL;

2. SELECT * FROM customer EXCEPT SELECT * FROM order_table;

3. SELECT * FROM customer c LEFT JOIN order_table o ON c.customer_id = o.customer_id WHERE o.order_id IS NULL;

4. SELECT * FROM customer WHERE order_id IS NULL;

1. SELECT * FROM customer WHERE age > (SELECT AVG(age) FROM customer);

2. SELECT * FROM customer WHERE age > AVG(age);

3. SELECT * FROM customer HAVING age > AVG(age);

4. SELECT * FROM customer WHERE age IN (AVG(age));

1. SELECT order_id, customer_id, LAG(order_id) OVER (PARTITION BY customer_id ORDER BY order_date) AS prev_order_id FROM order_table;

2. SELECT order_id, customer_id, MAX(order_id) FROM order_table;

3. SELECT order_id, customer_id, order_id-1 AS prev_order_id FROM order_table;

4. SELECT order_id, customer_id, MIN(order_id) OVER () AS prev_order_id FROM order_table;

SELECT category_name, AVG(price) 
FROM category c JOIN product p ON c.category_id = p.category_id 
GROUP BY category_name HAVING AVG(price) > 200;

SELECT order_id FROM order_table o

JOIN customer c ON o.customer_id = c.customer_id WHERE c.country_code = 'RU'

UNION ALL

SELECT order_id FROM order_detail WHERE product_id IN (SELECT product_id FROM product WHERE category_id = 1);

SELECT c.first_name, c.last_name, o.order_id FROM customer c

[INNER/LEFT/CROSS/ FULL] JOIN order_table o ON c.customer_id = o.customer_id;