CREATE TABLE products (product_id VARCHAR(255)); CREATE TABLE CHARACTERISTICS (characteristic_name VARCHAR(255), characteristic_id VARCHAR(255)); CREATE TABLE product_characteristics (product_id VARCHAR(255), characteristic_id VARCHAR(255))
What are characteristic names used at least twice across all products?
    
SQL: 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 HAVING COUNT(*) >= 2