方法一：使用子查询

• 首先，能将OrderItems表和Orders表关联起来的字段为order_num；
• 根据题目要求，需要先使用子查询来查询OrderItems，查询条件的字段为prod_id，查询的结果集为order_num字段的；
• 之后外层查询，查询Orders表，查询的条件为order_num字段，用order_num字段去匹配子查询的结果集；
• 最后再排序。

select cust_id, order_date
from Orders 
where order_num in (select order_num from OrderItems where prod_id = "BR01")
order by order_date;

方法二：使用2表连接

• 将OrderItems表和Orders表连接成新表，连接条件的字段为order_num；

select o.cust_id as cust_id, o.order_date as order_date
from OrderItems oi join Orders o on oi.order_num=o.order_num
where oi.prod_id='BR01'
order by order_date;

• 从group by字段开始，可以使用别名。

采用连接好像要简单高效一点。