SQL仅加入最大行拼图

Question

给定以下示例数据:

  Users
+--------------------------------------------------+
| ID | First Name | Last Name | Network Identifier |
+--------------------------------------------------+
| 1  | Billy      | O'Neal    | bro4               |
+----+------------+-----------+--------------------+
| 2  | John       | Skeet     | jsk1               |
+----+------------+-----------+--------------------+

 Hardware
+----+-------------------+---------------+
| ID | Hardware Name     | Serial Number |
+----------------------------------------+
| 1  | Latitude E6500    | 5555555       |
+----+-------------------+---------------+
| 2  | Latitude E6200    | 2222222       |
+----+-------------------+---------------+

 HardwareAssignments
+---------+-------------+-------------+
| User ID | Hardware ID | Assigned On |
+-------------------------------------+
| 1       | 1           | April 1     |
+---------+-------------+-------------+
| 1       | 2           | April 10    |
+---------+-------------+-------------+
| 2       | 2           | April 1     |
+---------+-------------+-------------+
| 2       | 1           | April 11    |
+---------+-------------+-------------+

我想编写一个 SQL 查询，它会给出以下结果:

+--------------------+------------+-----------+----------------+---------------+-------------+
| Network Identifier | First Name | Last Name | Hardware Name  | Serial Number | Assigned On |
+--------------------------------------------------------------------------------------------+
| bro4               | Billy      | O'Neal    | Latitude E6200 | 2222222       | April 10    |
+--------------------+------------+-----------+----------------+---------------+-------------+
| jsk1               | John       | Skeet     | Latitude E6500 | 5555555       | April 11    |
+--------------------+------------+-----------+----------------+---------------+-------------+

我的麻烦是需要为每个用户选择每个用户的最大“分配日期”并将其用于实际加入...

在 SQL 中有没有聪明的方法来完成这个？

Answer 1

SELECT U.NetworkIdentifier, U.FirstName, U.LastName,
       H.HardwareName, H.SerialNumber
  FROM (SELECT UserID, MAX(AssignedOn) LastAssignment
          FROM HardwareAssignments
         GROUP BY UserID) AS T
  JOIN HardwareAssignments AS HA
       ON HA.UserId = T.UserID AND HA.AssignedOn = T.LastAssignment
  JOIN Users AS U ON U.ID = HA.UserID
  JOIN Hardware AS H ON H.ID = HA.HardwareID
 ORDER BY U.NetworkIdentifier;

这和 Justin Niessner 的答案之间的区别在于子查询出现的位置；在这里，我在 FROM 子句中创建了它。这几乎可以保证它被执行一次。当 Justin 的回答中的 WHERE 子句中有一个相关的子查询时，优化器可能会为每一行执行一次子查询——当表很大时，这会更昂贵。一个真正优秀的优化器可能会将事物扁平化，使两者等价。