What are the sales made by the games in Japan region?
sales = SUM(num_sales); Japan region refers to region_name = 'Japan';
SELECT SUM(CASE WHEN T2.region_name = 'Japan' THEN T1.num_sales ELSE 0 END) AS nums FROM region_sales AS T1 INNER JOIN region AS T2 ON T1.region_id = T2.id