SQL加入到最大排拼图

给出以下示例数据：

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,这将产生以下结果：

+--------------------+------------+-----------+----------------+---------------+-------------+
| 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,每个用户需要为每个用户选择最大的“分配开”日期,并用于实际连接…

有没有一个聪明的方式在sql中完成这个？

解决方法

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;

这与Justin Niessner的答案之间的区别在于子查询出现在哪里在这里,我已经在FROM子句中创建了它.这几乎保证它被执行一次.当在Justin答案中的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 – 这在表很大的时候更昂贵.一个非常好的优化器可能会使事情变得平坦,以便两者相当.

原文地址：https://www.jb51.cc/mssql/75421.html