SELECT prod_name,
COUNT(order_num) AS orders
FROM Products p
LEFT JOIN OrderItems oi
ON p.prod_id = oi.prod_id
GROUP BY prod_name
ORDER BY prod_name ASC;
# 要求无实际订单的产品soda也返回，因为MYSQL不支持外连接，所以用左连接
# 左连接结果如下
# a0013 coffee a0013 a1121
# a0003 cola a0003 a5
# a0003 cola a0003 a19
# …
# 聚合计数即可