通过匹配 SQL Server 中的两列对结果进行分组 [英] Group result by matching two columns in SQL Server

问题描述

我使用的是 SQL Server 2008 R2.我有一个名为 Messages 的表，我在其中存储每个用户发送给其他用户的用户消息.表结构如下.

I am using SQL Server 2008 R2. I have a table called Messages where I store user messages each user send to other users. The table structure is like below.

+--------+----------+-----------------+------------+
| Sender | Receiver |     Message     |    Date    |
+--------+----------+-----------------+------------+
| John   | Dennis   | How are you     | 2015-06-06 |
| John   | Dennis   | Hi              | 2015-06-05 |
| Tom    | John     | How much is it? | 2015-06-04 |
| Tom    | John     | Did you buy it? | 2015-06-03 |
| Robin  | Tom      | Hey man         | 2015-06-03 |
| Dennis | John     | What up         | 2015-06-02 |
| John   | Tom      | Call me         | 2015-06-01 |
+--------+----------+-----------------+------------+

我想为每个对话获取选定用户的最新消息和其他参与者的姓名.例如，有三个对话.一个在john-Dennis"之间，第二个是John-Tom"，第三个在Robin-Tom"之间.

I want to get the newest message and other participants' name for a selected user for each conversation. For a example there are three conversations. One is between "john-Dennis" and 2nd one is "John-Tom"and 3rd one between "Robin-Tom".

如果我想获取用户 john 的对话，我想获取包含对话中其他用户姓名的最新对话消息.

If I want to get conversations for user john, I want to get the latest conversation message with the name of other user who is in the conversation.

上述场景的预期结果应该是这样的.

The expected result for the above scenario should be like this.

+-------------+-----------------+------------+
| Participant |     Message     |    Date    |
+-------------+-----------------+------------+
| Dennis      | How are you     | 2015-06-06 |
| Tom         | How much is it? | 2015-06-04 |
+-------------+-----------------+------------+

如何在 SQL Server 中使用 SQL 查询来实现这一点.我挣扎了好几天.请帮忙.提前致谢.

How to achieve this using a SQL query in SQL Server. I am struggling with part for days. Please help. Thanks in advance.

推荐答案

可以稍微压缩一下，但我已将其拆分为简单的步骤，希望能让它更容易理解.

It's possible to compress this a bit, but I've split it into simple steps to hopefully make it a little easier to follow.

-- Sample data from the question.
declare @msg table (Sender varchar(32), Receiver varchar(32), [Message] varchar(max), [Date] date);
insert @msg
    (Sender, Receiver, [Message], [Date])
values
    ('John','Dennis', 'How are you', '2015-06-06'),
    ('Dennis', 'John', 'Hi', '2015-06-05'),
    ('Tom', 'John', 'How much is it?', '2015-06-04'),
    ('Tom', 'John', 'Did you buy it?', '2015-06-03'),
    ('Robin', 'Tom', 'Hey man', '2015-06-03'),
    ('Dennis', 'John', 'What up', '2015-06-02'),
    ('John', 'Tom', 'Call me', '2015-06-01');

-- The name of the user whose conversations you want to find.
declare @UserName varchar(32) = 'John';

-- Step 1: Create columns [Participant1] and [Participant2] that will be the same for
--         each pair of users regardless of who's the sender and who the receiver.
with NameOrderCTE as
(
    select 
        Participant1 = case when Sender < Receiver then Sender else Receiver end,
        Participant2 = case when Sender < Receiver then Receiver else Sender end,
        *
    from
        @msg
),

-- Step 2: For each distinct pair of participants, create a [Sequence] number that 
--         puts the messages in reverse chronological order.
MessageSequenceCTE as
(
    select
        *,
        [Sequence] = row_number() over (partition by Participant1, Participant2 order by [Date] desc)
    from
        NameOrderCTE
)

-- Step 3: Get the most recent ([Sequence] = 1) messages for each conversation
--         involving the target user.
select
    Participant = case @UserName when Sender then Receiver else Sender end,
    [Message],
    [Date]
from
    MessageSequenceCTE
where
    @UserName in (Sender, Receiver) and
    [Sequence] = 1;

这篇关于通过匹配 SQL Server 中的两列对结果进行分组的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！