请参阅SQL Fiddle with Demo

Question

我正在尝试为邮件系统制定SQL查询，该查询将返回状态为“OPEN”且其最后一条消息未被某个组中的用户发布的所有线程的列表。

查询中涉及的表是：

threads
-------
threadId
timestamp
subject
status
clientId

messages
--------
messageId
userId
messagebody
timestamp
threadId

users
-----
userId
username
groupId

当前查询是：

SELECT threadId, timestamp, subject, status, clientId 
FROM threads
WHERE status='OPEN' 
ORDER BY timestamp DESC

工作正常;但我现在有一个新的要求 - 如果线程中的最后一条消息是由groupId为1的用户发布的，那么它不应该在结果集中。

包括我需要的信息很简单：

SELECT threadId, timestamp, subject, status, clientId 
FROM threads
INNER JOIN messages USING(threadId)
INNER JOIN users USING(userId)
WHERE status='OPEN' 
GROUP BY threadId
ORDER BY timestamp DESC

但此时我陷入困境，因为我无法弄清楚如何根据特定组中messages.userId timestamp的{{1}}来筛选线程。我已经查看了HAVING以及所有聚合函数，似乎没有任何内容符合要求。

修改

我可能不够清楚，所以我将尝试用一个例子说明。

以下是一些示例数据：

线程

threadId | timestamp | subject | status | clientId
--------------------------------------------------
1        | 1         | One     | OPEN   | 1
2        | 10        | Two     | OPEN   | 4
3        | 26        | Three   | OPEN   | 19
4        | 198       | Four    | OPEN   | 100

讯息

messageId | userId | messagebody | timestamp | threadId
-------------------------------------------------------
1         | 3      | Hello, World| 1         | 1
2         | 1      | Hello       | 3         | 1
3         | 1      | ^&%&6       | 10        | 2
4         | 2      | Test        | 12        | 2
5         | 4      | Hi mum      | 26        | 3
6         | 1      | More tests  | 100       | 4

用户

userId | username | groupId
---------------------------
1      | Gareth   | 1
2      | JimBob   | 2
3      | BillyBob | 2
4      | Bod      | 3

在此示例中，数据集线程1和4应该从查询中删除，因为用户Gareth（组1的成员）是最后一个发布给它们的（threadId 1中的messageId 2和messageId 5 in threadId 4）。因此，结果集应该只包含线程2和3的线程数据（threadId，timestamp，subject，status和clientId）。

我为此测试数据创建了SQLfiddle。

有人能指出我在正确的方向吗？

Answer 1

听起来你想要这个：

SELECT threadId, timestamp, subject, status, clientId 
FROM threads t
INNER JOIN messages m
  ON t.threadId = m.threadId
INNER JOIN users u
  ON m.userId = u.userId
  and u.groupId != 1  --placed the groupId filter here.
WHERE status='OPEN' 
GROUP BY threadId
ORDER BY timestamp DESC

编辑，这似乎可以满足您的需求：

SELECT t.threadId, 
  t.timestamp, 
  t.subject, 
  t.status, 
  t.clientId
FROM threads t
INNER JOIN messages m
  ON t.threadId = m.threadId
INNER JOIN users u
  ON m.userId = u.userId
WHERE status='OPEN' 
  AND NOT EXISTS (select t1.threadid
                  FROM threads t1
                  INNER JOIN messages m
                    ON t1.threadId = m.threadId
                  INNER JOIN users u
                    ON m.userId = u.userId
                  where u.groupid = 1
                     and t.threadid = t1.threadid)
ORDER BY timestamp DESC

请参阅SQL Fiddle with Demo

Answer 2

~~在查询中添加LIMIT 1是否可以解决您的问题？~~

SELECT threadId, timestamp, subject, status, clientId FROM threads INNER JOIN messages USING(threadId) INNER JOIN users USING(userId) WHERE status='OPEN' GROUP BY threadId ORDER BY timestamp DESC LIMIT 1

或将where子句更改为

WHERE status='OPEN' and groupID<>1

<击>

我认为这就是你想要的？

select * from threads where threadId not in ( select messages.threadId from messages inner join (select threadId, MAX(timestamp) maxtime from messages group by threadId) t on messages.threadId = t.threadId and messages.timestamp = t.maxtime where messages.userId=1 )

如何使用GROUP BY子句过滤SQL查询

2 个答案:

请参阅SQL Fiddle with Demo