``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
               
  
               
               
               
               第一次用count(*)就错了，因为count(*)中计算order_num有个为null没有返回0，反而返回了1
    所以要用count(order_num) 忽略null值