Which product have they complained about? | Who do you mean by "they"? | Customer, 113.
SELECT DISTINCT t1.product_name FROM products AS t1 JOIN complaints AS t2 ON t1.product_id   =   t2.product_id JOIN customers AS t3 GROUP BY t3.customer_id having t3.customer_id =  "113"
product complained about refers to product_name; 'they' refers to a specific customer identified by customer_id = 113; complaints linked to products and customers through product_id and customer_id respectively;