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[英]How can I include a field that is not part of the group clause in a ActiveRecord query?
[英]How can I filter an SQL query with a GROUP BY clause
我正在尝试为消息传递系统制定一个SQL查询,该查询将返回状态为“OPEN”且其最后一条消息未由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具有最高timestamp
的messages.userId
来过滤掉线程的查询。 我已经看过HAVING
和所有聚合函数,似乎没有任何东西符合要求。
编辑
我可能不够清楚,所以我将尝试举例说明。
这是一些示例数据:
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
消息
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threadId 4中的messageId 5)。 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)。
我为这个测试数据创建了一个SQLfiddle 。
任何人都能指出我在正确的方向吗?
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
在查询中添加
LIMIT 1
可以解决您的问题吗?
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
或者将where子句更改为
WHERE status='OPEN' and groupID<>1
我想这就是你想要的?
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 )
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