How strong is the Hulk?
how strong refers to attribute_value WHERE attribute_name = 'Strength'; the Hulk refers to superhero_name = 'Hulk';
SELECT T2.attribute_value FROM superhero AS T1 INNER JOIN hero_attribute AS T2 ON T1.id = T2.hero_id INNER JOIN attribute AS T3 ON T2.attribute_id = T3.id WHERE T1.superhero_name = 'Hulk' AND T3.attribute_name = 'Strength'