Question: Calculate how many percent of customers were located in India.
SQL: SELECT CAST(SUM(IIF(T1.country = 'India', 1, 0)) AS REAL) * 100 / COUNT(T4.customer_id) FROM country AS T1 INNER JOIN city AS T2 ON T1.country_id = T2.country_id INNER JOIN address AS T3 ON T2.city_id = T3.city_id INNER JOIN customer AS T4 ON T3.address_id = T4.address_id