Question

我想要实现的是非常基本的，我找到了类似的解决方案here，但我无法使用我正在使用的MS SQL Compact。

假设我有这些表：

User                    Comments
ID   | Name             UserID | Comment           | Posted
-----+-------------     -------+-------------------+----------
0    | Adam             1      | Whatever 1        | 2014-01-01
1    | Beth             1      | whatever 2        | 2014-01-02
2    | Chris            0      | Whatever 3        | 2014-01-02
3    | David            3      | Whatever 4        | 2014-01-04

我想要的是获取所有用户并提供他们的最新评论（如果有的话）。如果没有评论它应该只是为空。也就是说，我想要这个结果：

Name  | Comment          | Posted
------+------------------+-----------
David | Whatever 4       | 2014-01-04
Beth  | Whatever 2       | 2014-01-02
Adam  | Whatever 3       | 2014-01-02
Chris | NULL             | NULL

到目前为止，我所拥有的是类似于此代码的代码（本例简化）：

SELECT u.Name AS Name, c.Comment AS Comment, c.Posted AS Posted
FROM User AS u
INNER JOIN Comments AS c ON u.ID = c.UserID
INNER JOIN
(
    SELECT UserID, MAX(Posted) AS Posted FROM Comments AS c2 GROUP BY UserID
) AS c2 ON c2.Posted = c.Posted ORDER BY Posted DESC

这在某种程度上有效，但在我的例子中，我有100个用户和不同的UserID：在评论中有93个帖子。但我只有84行，当我应该有100行，7行空行？所以，我的问题是：

1）我的查询有什么问题

2）你会怎么做？

Answer 1

您的查询是在正确的轨道上，但您需要left join而不是inner join并重新排列加入顺序：

SELECT u.Name AS Name, c.Comment AS Comment, c.Posted AS Posted
FROM User u LEFT JOIN
     (SELECT UserID, MAX(Posted) AS Posted
      FROM Comments c2
      GROUP BY UserID
     ) c2
     ON u.ID = c2.UserID LEFT JOIN
     Comments c
     ON c2.Posted = c.Posted
ORDER BY Posted DESC;

您的查询仅返回至少有一条评论的用户，因此您会错过没有评论的用户。

Answer 2

这与戈登·林诺夫所说的相同，除了最后一次左联盟。

    SELECT u.Name AS Name, c2.Posted AS Posted,c.Comment
     FROM [User] u LEFT JOIN
     (SELECT UserID, MAX(Posted) AS Posted
      FROM Comments c2
      GROUP BY UserID
     ) c2
     ON u.ID = c2.UserID LEFT JOIN
     Comments c on c2.Posted=c.Posted AND c2.UserId=c.UserId

从其他表中选择MAX值（如果有）

2 个答案: