Question

I’m trying to formulate an SQL query for a messaging system that will return a list of all threads which have a status of ‘OPEN’ and whose last message was not posted by a user in a certain group.

The tables involved in the query are:

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

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

users
-----
userId
username
groupId

The current query is:

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

which works fine; but I now have a new requirement – if the last message in the thread was posted by a user with a groupId of 1 then it should NOT be in the result set.

Including the information I need is simple enough:

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

but at this point I get stuck, because I can’t figure out how to get the query to filter out threads based on the messages.userId with the highest timestamp within a particular group. I’ve looked at HAVING and all the aggregate functions and nothing seems to fit the bill.

Edit

I may not have been clear enough, so I’ll try to illustrate with an example.

Here’s some example data:

threads

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

messages

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

users

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

In this example dataset threads 1 and 4 should be eliminated from the query because user Gareth (a member of group 1) is the last one to post to them (messageId 2 in threadId 1, and messageId 5 in threadId 4). So the result set should only have the thread data (threadId, timestamp, subject, status and clientId) for threads 2 and 3.

I’ve created an SQLfiddle for this test data.

Can anyone point me in the right direction here?

Answer 1

Does adding LIMIT 1 to your query solve your problem?

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

Or changing the where clause to

WHERE status='OPEN' and groupID<>1

I think this is what you want?

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
)