What is the ratio between business in shopping category and business in pets category?
ratio = divide(count(business_id where category_name = 'Shopping'), count(business_id where category_name = 'Pets'))
SELECT CAST(SUM(CASE WHEN T2.category_name = 'Shopping' THEN 1 ELSE 0 END) AS REAL) / SUM(CASE WHEN T2.category_name = 'Pets' THEN 1 ELSE 0 END) AS radio FROM Business_Categories AS T1 INNER JOIN Categories AS T2 ON T1.category_id = T2.category_id