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;
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