SQL - 如何根据生效日期获取最新价格？

Question

我无法回答非常直接的问题。如何根据最新生效日期（给定 7-6-2020 作为生效日期）获得每种产品的有效成本？

价格Table：

id  | product_name  | cost   | effective_date 
=======================================================
1   | Product A     | 8.00   | 1-5-2020
1   | Product A     | 9.50   | 1-6-2020
1   | Product A     | 10.00  | 1-7-2020
2   | Product B     | 4.00   | 5-6-2020
2   | Product B     | 4.50   | 5-7-2020

预期结果：

id    | product_name  | cost   | effective_date 
-----------------------------------------------
1     | Product A     | 10.00   | 1-7-2020     
2     | Product B     | 4.00    | 5-6-2020    

Answer 1

一种方法是相关子查询：

select t.*
from t
where t.effective_date = (select max(t2.effective_date)
                          from t t2
                          where t2.id = t.id and
                                t2.effective_date <= '2020-07-06'
                         );