SQLiteStudio：从价格目录获取最新价格

我对SQL相当陌生，正在尝试获取特定日期的产品交易价格，我在价格目录中查找交易之前该产品的最新价格

具体而言，我有以下两个表格：

Transactions                                Catalog 
----------------------------------------------------------------------------
ProductID | Design     | Transaction_DT     ProductID | Price | Effective_DT

1         | Plaid      | 5/14/2016          1         | 20    | 4/22/2016
2         | Solid      | 3/26/2016          1         | 10    | 5/2/2016
3         | PolkaDot   | 4/12/2016          1         | 5     | 5/15/2016
4         | Solid      | 4/24/2016          2         | 50    | 3/22/2016
5         | PolkaDot   | 2/24/2016          2         | 25    | 4/1/2016
6         | PinStripe  | 3/29/2016          2         | 10    | 4/2/2016
                                            3         | 30    | 4/5/2016
                                            3         | 25    | 4/9/2016
                                            3         | 22    | 4/12/2016
                                            4         | 12    | 3/15/2016
                                            4         | 8     | 3/27/2016
                                            4         | 6     | 4/25/2016
                                            5         | 15    | 2/23/2016
                                            5         | 11    | 2/25/2016
                                            5         | 6     | 2/28/2016
                                            6         | 26    | 2/2/2016
                                            6         | 17    | 3/19/2016
                                            6         | 13    | 5/16/2016

我已输入以下代码：

SELECT Transactions.ProductID,
       Catalog.Price,
       Transactions.Transaction_DT,
       Transactions.Design
  FROM Transactions
       LEFT JOIN
       Catalog ON Transactions.ProductID = Catalog.ProductID AND
       Catalog.Effective_DT = (
                                  SELECT MAX(Effective_DT)
                                    FROM Catalog
                                   WHERE Effective_DT <= Transactions.Transactions DT
                              )

---

我想返回产品1、4和6的价格分别为10、8和17，除了正确输出的正确价格，而不是我得到的空值。有没有关于如何获得正确结果的想法？

您忘记了按productID筛选相关查询。您没有获得产品的正确最新日期。您需要使用此查询：

SELECT Transactions.ProductID,
       Catalog.Price,
       Transactions.Transaction_DT,
       Transactions.Design
  FROM Transactions
       LEFT JOIN
       Catalog ON Transactions.ProductID = Catalog.ProductID AND
       Catalog.Effective_DT = (
                                  SELECT MAX(Effective_DT)
                                    FROM Catalog
                                   WHERE Effective_DT <= Transactions.Transactions_DT 
                                     and ProductID = Transactions.ProductID
                              )

---

这不仅仅是一个价格。。你想要最新的吗。？老的？

SELECT Transactions.ProductID,
       Catalog.Price,
       Transactions.Transaction_DT,
       Transactions.Design
  FROM Transactions
       LEFT JOIN
       Catalog ON Transactions.ProductID = Catalog.ProductID AND
       Catalog.Effective_DT = (
                                  SELECT MAX(Effective_DT)
                                    FROM Catalog
                                   WHERE Effective_DT <= Transactions.Transactions_DT 
                                     and ProductID = Transactions.ProductID
                              )