How many customers are there?
select count ( * ) from customers
Yes, please.
select customer_id, count ( * ) from complaints group by customer_id
Can you tell me the id of the product with most complaints?
select product_id from complaints group by product_id order by count ( * ) desc limit 1
What are the emails of the customers who filed a complaint of this product?
select t1.email_address from customers as t1 join complaints as t2 on t1.customer_id = t2.customer_id where t2.product_id = 117