Can you please show me the characteristic name used by least number of products?
SELECT t3.characteristic_name FROM products AS t1 JOIN product_characteristics AS t2 ON t1.product_id   =   t2.product_id JOIN CHARACTERISTICS AS t3 ON t2.characteristic_id   =   t3.characteristic_id GROUP BY t3.characteristic_name ORDER BY count ( * )  LIMIT 1
characteristic name used by the least number of products refers to MIN(COUNT(characteristic_name));